Electrons' mass is a little strange, in my (limited) understanding. The Standard Model predicts that electrons should be massless, but they gain mass (or maybe easier to say, "massiness") through interactions with the Higgs field. This effectively creates a constant "drag" on electrons, giving them a kind of inertia or "mass". I think this video does a good job explaining it, much better than I could: https://youtu.be/kixAljyfdqU