Municipal surplus (fund balance)
What a town has left when its cash and other quick assets are greater than its liabilities. A town can use some of it as revenue in next year's budget, which lowers the amount it must raise from property taxes.
New Jersey example
Made-up numbers, simplified: a town plans $30 million of spending and expects $8 million from other revenue. If it uses $2 million of surplus, it must raise $20 million from property taxes instead of $22 million. Unless the state Director agrees, the surplus used can't be more than the town held in cash or quick assets at the start of the year.
Official source
This is a plain-English summary. Rules and dollar amounts change, so check the official source or a professional before you file or sign anything.