Ironically, an internet search for images of "foreplay" usually brings up pornographic pictures involving only bodies, which is strange -- because foreplay is really the process of people (not just bodies) preparing for sexual union. The whole idea of "play," after all, suggests a relationship between people.
Foreplay is a process that covers a lot of ground: it often begins with both husband and wife in a state of sexual interest that is "pre-arousal" -- they are not yet physiologically and psychologically aroused. Foreplay is the process of moving from the pre-arousal state to increasing levels of arousal, until the bodies (and emotions) are completely prepared for sexual union. That process can be very quick, very slow, or somewhere in between. An important part of foreplay is the partners accommodating each other in their foreplay preferences (both in general and for that specific act).
A fairly sensible description of foreplay can be found on the Full Wiki on the subject: "In human sexual behavior, foreplay is a set of intimate psychological and physical acts between two or more people meant to create and increase sexual arousal, in anticipation of sexual intercourse. Any act that creates and enhances sexual stimulation between the sex partners may constitute foreplay, including kissing, touching, embracing, talking, and teasing . . ."
Unfortunately, many written discussions of foreplay on the internet are formulated in terms of what one person "does to" the other. (The "moves" that will drive him, or her, "wild"!) But there are also a good number of sensible discussions of foreplay as well. The reason is simple: a lot of women write on the web, and it is a permanent frustration of women from time immemorial that men so often want to "just do it" -- that is, they want to move right to the physical manipulation of the bodies to stimulate and enjoy physical pleasure. Women, on the other hand, are more inclined to want an extended period of embracing and kissing and caressing and talking (an exchange of affectionate, romantic words) prior to really "getting into it" physically. That psychological dimension is central to foreplay.
Part of the reason for women's need for foreplay is physical: women are "made" to need time to get aroused and have their bodies prepared for sexual intercourse (most obviously, by the natural lubrication of the sexual organs -- although this may be as good a place as any to point out that artificial lubrication is a wonderful thing, and helps both partners experience greater pleasure in the caresses of either the vagina/clitoris or the penis).
But the reason for the need for more attention to foreplay is also psychological. Women tend to focus more on the personal relationship in sex, compared to men, and foreplay is a way of cultivating that sense of personal union that is so important to them.
That's why "foreplay" doesn't consist simply in caressing a woman's breasts or vulva -- her most obvious "erogenous" zones, which men tend to latch onto quickly -- but in caressing the whole body and also in the affectionate words accompanying the kisses and caresses.
A husband needs to tell his wife that he admires her, and why he admires her. He needs to know what her psychological needs are: her worries, her frustrations, her desires and aspirations -- and what makes her feel loved, what makes her feel close to, united with this man with whom she is making love. A man has to create a spiritual union with his wife to prepare her for the physical union.
Men tend to get impatient ("yeah, yeah, I know we gotta talk"), partly because the sexual urge is propelling them forward at a faster pace. But, of course, they can learn to be more patient, and the "payoff" is great: their beloved will ultimately respond more powerfully to their lovemaking. And some men even learn to enjoy the foreplay as much as their wives (though probably not the "foretalk")!
This is one area in which it is particularly important not to misundertand the traditional moral ideal of "doing unto others as you would have them do unto you." That doesn't necessarily mean doing to a woman what you, as a man, would like done to you. Men, when it comes to sex, generally tend to be very "phallocentric" -- they focus pretty heavily on what is being done (or not being done) to their penis. Sometimes they assume, unthinkingly, that a wife likewise wants her husband to focus on her genitals. Husbands do this at least partly because, once a wife is really aroused, she really does like that attention to the genitals. But that arousal takes a pretty good while (at least for most wives). This isn't really news -- it's pretty well known that women really value their entire body being involved in foreplay, with the concentrated focus on the genitals put off until they have become very aroused.
It's good to keep in mind here what I have noted in another section (here): that many women (especially those in long-term monogamous relationships) don't start with (physical or sensual) sexual desire that leads them to engage in sexual relations (as is typical with men). Wives often start with a (non-physical) desire to express love for their spouses or to achieve emotional closeness, which leads them to engage in sexual activity, which then brings on sexual desire.