How does Forward Error Correction actually work? Does it?

A lot of products offer Forward Error Correction. This is most commonly uttered in the Wan Optimization space, as well as the SD-WAN space... but there's some stand alone products that do so as well. For example the voip codec "OPUS" says it has Forward Error Correction built into the protocol.

I'm thinking if so many "things" have this, it must be real. But it sounds like impossible nonsense. Basically packet loss across the wan can be "fixed" on the other side, and the lost packets restored. How, I don't know, but that's why I'm asking.