What is really happening when the baby kicks? What causes this? How often should I feel the baby kicking?
Babies kick if they are healthy just to get their exercise. Humans are not motionless by nature! If you have an opportunity to be around a newborn, you will see that they are constant-motion machines unless they are sleeping, and many of them are even when they are asleep!
As for how often they should kick, that really varies depending on the baby. One of the fun things about being pregnant is to get to know your baby before s/he is even born. You will probably know when your baby is most active and when your baby sleeps. Some providers like you to do "kick counts" in later pregnancy, and keep track of how long it takes to get 10 kicks during a "busy" time.
I tell my clients that if their baby is not acting like they usually do ("my baby is always jumping on a trampoline at 11 p.m.!), to drink some orange juice, lie on their side and pay attention to their baby. If it really isn't doing anything at all, they need to call me.
Bur remember that at the end of pregnancy, the baby is running out of room in there, so there may be more "squirming" than "kicking," which also counts!