it's the distance between the studs in your rims. Mine is 4x100, so only 4x100pcd wheels will fit on my car. Some rims also have multi fit (like those black ones) and can fit on multiple pcd's. You either have 4x100 or 4x114.3 pcd. And the multi fit wheels have both pcd's available (you can tell because they have a lot of stud holes drilled into them, such as the rims you posted just before). I think the multi fit on 5stud rims is 5x114.3 and 5x120 on most... sometimes 5x100.
It's a lot to try and understand right away, but you get used to it.
Easy way to figure it out:
10 studs = multi-fit for 5stud wheels
8 studs = multi-fit for 4stud wheels
5 studs = 5stud wheel, need to check pcd to make sure it fits
4 studs = 4stud wheel, need to check pcd to make sure it fits