Skill Points are points that can be used for upgrading your skills or making your shield stronger. Players can upgrade a class' skill with skill points or use them to upgrade their shield level. You can get 5 skill points after winning three consecutive matches or win some as a reward from events. If you don't want to wait till you'll get those and want to max your skills and shield immediately, can you use diamonds, but the cost will get more expensive every time you upgraded the skills/shield with diamonds and after upgrading both of those, can't you use them for anything anymore.
There are 8 skills per class the first tier are 2x 5 upgrades and cost 10 skill points per upgrade, the second tier has got 2x 10 and cost 20 skill points per upgrade, the third tier has got 2x 16 and cost 30 skill points per upgrade and the last tier has got 2x 25 and cost 40 skill points per upgrade.
You could also take back all your skill points from the skills menu for 599 diamonds and use them for another tier or your shield, but that's not recommended to do. However this feature has been removed in update 16.