What does electricity do when you unlock it at level 10?

It is a type of resources and also to upgrade your troops

Electricity and metal are advanced types of resource which happens to be used in addition to paint and plastic, for you to be able to train troops (in their corresponding facilities) level 10 and up. Similarly, you also get to build the specific resource collectors at the same time (e.g. "battery facility" to generate electricity). For the same reason, you wont be able to collect these resources in the battlefield unless your HQ is level 10+.