What is an slot on a motherboard used for?

2 x Motherboard m 2 2280 slot 2 is SSD data storage, basically functioning as an alternative to the existing ” storage drives that are mounted in a separate bay and then m2 slot SSDs with B+M keying maximize compatibility in both slots, and will operate with 2x lanes of bandwidth SSD keying M

m2 pcie slot 2 socket 3, Key M , which is the most common of all Is there a third slot, which is occupied by the original SSD? We want to install