what is the "Banlist" (Bitcoin Core) Upon start up, after it has downloaded and verified the blocks, the Bitcoin Core client "loads the banlist". What is this banlist?
It is a blacklist of erroneous nodes' IP addresses which has accrued a certain amount of banscore due to "misbehaviour". To check what sort of erroneous behaviour is considered "misbehaving", you can check the Core source code.

It is a precaution to protect your node against potential denial of service attacks. Example actions which will accrue banscore includes spamming "mempool" messages, relaying invalid transactions/blocks, etc...