What does the sixth line in the output of the Bitcoin Core GUI RPC command "gettxoutsetinfo" mean?
It is a hash of the entire serialized UTXO set. You can use to verify that a node's UTXO set is uncorrupted (by comparing it with another node that runs the same command, at the same height).
The _2 prefix is to indicate the hashing algorithm that is used. Earlier versions used a different algorithm that became very hard to support efficiently. It's just there so people don't expect the hashes to be comparable between old and new nodes.