Positive reinforcement means encouraging a behaviour by responding to it with something pleasant — praise, attention, a reward or a privilege — which makes that behaviour more likely to happen again. In parenting, it's the principle behind 'catching children being good': noticing and rewarding the behaviour you want to see, rather than only reacting to the behaviour you don't.
Children naturally seek attention and approval, so behaviour that earns warm, positive responses tends to be repeated. Rewarding good choices (sharing, listening, trying hard) with specific praise and encouragement reinforces them far more effectively than nagging or punishment reinforces the avoidance of bad behaviour. It also builds a warmer relationship and a child's self-esteem, since the focus is on what they're doing right.
Reward and behaviour charts are a common way to apply positive reinforcement at home — see our free reward chart and behaviour chart, and our guide to reward charts that work. This is general information; for persistent behavioural concerns, your health visitor, GP or school can help.