Rebalancing means adjusting your portfolio back to your planned asset allocation when there is a shift due to market movements. Over a period, some assets might give very high returns, due to which there might be a change in your planned asset allocation. So, in such cases, it is better to book some profits and reinvest the money in assets to match the originally planned asset allocation mix. 

There’s no fixed rule for frequency but reviewing annually or every two years is suggested. Equity investments often don’t need rebalancing in the first 5–6 years unless there are major market changes.