To "socialize" an animal means to teach it to be "friendly" to people.  This makes the animal more manageable to care for and more adoptable, a plus if the cat ends up in the shelter where cats labeled as "feral" are routinely killed.

   The younger an animal is, the easier it is for it to be socialized.            After 8 weeks old, the socialization "window" starts to close and the difficulty of success increases as the cat ages.  It is possible to take kittens as young as 5 weeks to wean and socialize, and in cases of bottle-fed, motherless kittens, even younger. 
   Taken kittens can be kept in a small safe room or kitty condo and all food experiences are tied to human contact. Outside cats and kittens can be "lured" closer and closer by tossing food to them or  just patiently hanging out and waiting to get a little closer and closer. The ultimate goal being the cat being comfortable with handling and sitting/eating on a person's lap, acheived by the cat/s learning that the food will only come if they are "friendly."

   More ambitious caretakers can learn on-line or from books about how to "clicker-train," and fun cat tricks they can do with food.