Locked By lets you click the Lock or In Use icon to find out why a record is locked.
You might know you can click the Lock icon to see who is using a record, like a Customer, that is normally unlocked.
Locked By adds an explation for transactions like Invoice, which can be locked by Payments, being in a Locked Period, and being from a Closed Order.
Locked By Order - this is the explanation for a locked invoice from a Closed Order. If you partially invoice and then cancel the remaining order by marking it Closed, this locks the invoice to ensure consistency. To unlock the invoice, mark the order as Unclosed.