What are brain zaps

Something I know is very common, particularly among those who take antidepressants is a brain zap. It often occurs alongside a missed dose so I presume it’s something like a ‘withdrawal’ symptom.

So my question is, what is a brain zap, what’s happening on a molecular/cellular level?

EDIT: I know what they are and feel like - I have them a lot. I was more wondering the science behind it.