Ask HN: What makes a good explainer video?

Hello all,

I am launching an app soon and I am considering placing an explainer video on the app's website homepage.

My question is two fold; what makes a good explainer video? (and also is it still relevent?)

From memory, I remember lots of startups using them a couple of years back however now it appears the background image with explainer text/grahpic is preferred.