What does a Lock Unspent command do?
“Lock Unspent” works with the Coin control feature enabled ( Settings - Options - Wallet - Enable coin control) in the following way.
When you create a new transaction and chose inputs manually from Inputs (in Send menu), you can lock one of the inputs to avoid spending coins from it by accident.
NOTE: only the coins from one input can be locked at a time. Not from one address, but from a separate input, so you cannot select all inputs from the same address, e.g. linked to the bank, and lock the coins at it at the same time (at least not at the moment). You can select each input manually.
NOTE: that inputs are locked only for the wallet work time, i.e. as long as the wallet is open and running.