Gas refers to the unit that measures the amount of computational effort required to execute specific operations on the Polygon network.
Since each transaction requires computational resources to execute, each transaction requires a fee. Gas refers to the fee required to conduct a transaction on Polygon successfully.
Gas fees are paid in Polygon's native currency, matic (MATIC). Gas prices are denoted in gwei, which itself is a denomination of MATIC - each gwei is equal to 0.000000001 MATIC (10-9 MATIC). For example, instead of saying that your gas costs 0.000000001 MATIC, you can say your gas costs 1 gwei. The word 'gwei' itself means 'giga-wei', and it is equal to 1,000,000,000 wei. Wei itself (named after Wei Dai, creator of b-money) is the smallest unit of MATIC.
More on gas fees here.
Monitor gas prices, so you can send your transaction for less:
High Gas Fee - Red
Medium Gas Fee - White
Low Gas Fee - Green