I am looking at BIP 109. It said, "The existing MAX_SIGOPS limit of 20,000 signature operations per block shall be retained, but only ECDSA verifications actually performed to validate the block shall be counted." What do MAX_SIGOPS and signature operations stand for?
Signatures are how all bitcoins are proven to be able to be spent by the owner of an address. If you receive bitcoins to an address, that address will generally correspond to at least one (maybe more in a multi-signature address) ECDSA private key, which is used when you spend those bitcoins later. But for other nodes to verify signatures to make sure all transactions are valid, that takes work, so there is a limit of how many of these signature operations a block can contain. That makes sure a block doesn't put too much workload on nodes verifying it. That limit is the MAX_SIGOPS.