Optimistic locking: Where Access only tries to lock a record when someone tries to save any changes that they have made.
Optimistic locking implies that, when a user edits a record, Access optimistically assumes that noone else will want to edit the same record, and so it doesn't lock it.
Pessimistic locking: When Access locks a record whenever someone starts to change the contents of that record.
Pessimistic locking implies that, when a user edits a record, Access pessimistically assumes that someone else will want to edit the same record and so needs to lock it.