Happy scale confusion!

It's the average of your weight over the previous days. Depending on the default settings (I use libra rather than Happy Scale) it's usually 7 days so if you enter your weight daily Sunday's Moving average is the average of Mon - Sun on Mon it'd be Tues - Mon, and so on.

Depending on how much you fluctuate up and down, it's a fairly decent indication of your weight so you can work out what rate you're losing at over time.