What is your current motherboard and cpu?
Why do you need a new motherboard?
Mechanical - means physical size of slot. It can be x16, x4, x1.
Electrical - means pcie lanes that are electrically connected (some can be not connected).
Fot example, you can have x16 physical slot, where connection is only x8 or x4. So electrical connection will always be smaller or same as physical size of slot.
Here illustration:
upper x16 slot (long yellow) - x16 physical, x16 electrical.
lower 2 x16 slots (long black) - x16 physical, x8 electrical.
4 short yellow are x1 slots.