A spacer is a metal spacer that you place between the motherboard and the case.
How to Add/Remove Standoffs for Your PC Case/Motherboard [EASY!]
When building a PC, one of the most critical components you will install is your motherboard. On a motherboard, there are many electrical connections on the back of the board and the front of the board.
Since computer cases are made of metal, there must be space between the motherboard and the case so that they do not touch each other.
Standoffs are small pieces of metal that resemble screws. However, instead of a head, a standoff has an anchor into which another screw can be placed. Standoffs are traditionally screwed into a case. The motherboard is then placed on the standoffs and screwed into them.