An expansion slot is a socket on a computer motherboard that allows you to add additional components to your system. These slots are used to expand the capabilities of your computer and can be used to add new functionality to your system.
"Capping" a slot machine simply means locking it down. This is most often done by a slot attendant on behalf of a player at that machine who wants to take a break: go to the bathroom, step outside for some fresh air, get a quick snack, etc.