Nevermind about the G3258, the 3420 is pretty solid and near to identical in out of the box performance.
Expansion slots are the covered sections at the rear of a case, which line up with the slots on a motherboard which you can plug various items into.
A dual slot graphics card (like the one you picked) overhangs the slot below it, meaning the motherboard you want to use will not work, but most ITX boards will - basically anything on
this list
In case you need clarity -
Most "ITX" cases are sold with two expansion slots to cover a dual slot graphics card overhanging the motherboard edge. ITX motherboards can only ever accommodate one expansion slot on the board, usually a PCIe x16.
Example pic.
Most "mATX" cases are...