1. fill the position left by a departing employee
backfill a position after someone leaves the company
This is usually used in the context of recruitment. When a recruitment company has to find another employee to replace someone they had previously placed in the role it is referred to as 'backfilling'. To backfill means to replace workers after jobs have become available. In the construction industry, 'backfill' is when material that was first dug out of a hole is then returned or replaced to fill it up again. This term is also used in Archaeology. So, in the business world, you can see the analogy - a vacancy is left by the person who leaves, and it has to be filled.