This really is as it sounds: play before sex - though it doesn't have to lead to ful-on penetrative sex. Its biological importance is to prepare the body for sexual intercourse, even if that point is not reached every time you're together.
Stroking, kissing, licking, caressing, massaging, touching, holding and even eye contact can all make you and your partner become increasingly aroused. This is essential if you want the right physical, or bodily, and emotional conditions to have sex. Your body will become much more sensitive than normal, your heartbeat will increase - and you should feel great!
One thing that happens is the increase of blood flow to the genitals in both the male and female bodies. For a boy, the penis will become erect as blood enters the erectile tissue inside the penis and causes it to swell up in girth and length. For a girl, the vagina will produce a type of fluid or 'mucus' which will act as a form of lubrication during intercourse. The area around the vagina will swell, to provide a 'cushion' for during intercourse. The nipples and clitoris will also be affected: the nipples will harden while the clitoris begins to erect, just like the penis, and emerges from beneath the clitoral hood.
But not tacky. Say hi and ask what they're doing this weekend - and have a plan to suggest you think they'll like.
Relax and think: getting to know you. Rather than: getting it