Are you talking about a PC Case? Or the motherboard?
Expansion slots on the motherboard are used to connect devices like GPUs, Wireless adapters, high end Sound cards, etc.
Your Case (the box your PC goes in) has cutouts on the back which line up with those slots so you secure the add-in cards with screws and have access to the external connects on the back of the card.
A case with "7 slots" will almost certainly be a full sized ATX case, that can fit a standard sized ATX motherboard with all 7 slots available.
Unless you're building some server or mining workstation with very unusual requirements, a 7 slot case with ATX motherboard is completely standard and all you should ever need.