what is the difference between Two PCI Express* x1 bus add-in card connectors? and One PCI Express x16 bus add-in card connector?
the x1 slots are for expansion cards & the x16 for the video card
The x16 is a longer slot, with 16 “lanes” for a potential throughput of 4GB/s, the successor to the AGP slot of older boards (but capable of taking lower speed PCI-E cards as well).
The x1 slots have a potential 250MB/s throughput - and while they are the logical successor to PCI, with more throughput available to each card than to the entire PCI subsystem, they have only slowly gained support, leaving the now “legacy” PCI slots as still the most important after the graphics.
Some motherboards also implement a x4 slot (which can also be used as an x1).