When you purchase an NFT or transact on the blockchain, a fee known as "gas" is incurred. Every transaction ever made on a blockchain is recorded in a public ledger. Every time a new transaction is made, a complicated set of proofs are executed to validate the buyer and the seller and encode that new transaction into the public ledger.
All of this requires computing power. More traffic on the network means more computational power is needed to process a transaction. Gas fees are the costs to transact on the blockchain. Their price can vary materially from one blockchain to another.