The amount of money left in a charity’s bank account at the end of a year (after income and expenditure have been accounted for).

In a company structure this would be described as ‘profit’, but it’s more like a ‘savings account’ or a financial buffer, to be used in the event of unpredictable income caused by external events which negatively impact the charity’s ability to raise money (e.g. global pandemic)

Charities are expected to have enough in reserve to be able to meet the costs of winding up should they be unable to continue their work. 

This NCVO article explains reserves in more depth.