Motherboard Standoff
Most cases use threaded brass standoffs (Jack Screw Standoffs) for attaching the motherboard to the case chassis. Sometimes threaded or snap-lock plastic standoffs are used, which are less secure, but equally useful in a stationary computer. The standoff provides a margin of space between the motherboard and the case to keep the multiple solder points below from grounding and short-circuiting.
Read more about this topic: Computer Case Screws