Reinforcement is a term used in operant conditioning to refer to anything that increases the likelihood that a response will occur. If every time you stay home on Sunday night your child performs better on the tests on Monday, and you are more likely to stay home because of that, then your child performing well on Monday tests reinforces your staying home the night before. When other people reinforce you for being with them, you want to socialize more.
There are many reinforcers for behavior. Among the naturally occurring reinforcers, social reinforcement is one of the most commonly occurring. Social reinforcers come from other people and include smiles, hugs, praise and attention. Social reinforcers are powerful. Acceptance and approval of others means you are part of the group/family and communicate that you are a lovable person.