What's the consequence of the Advent completing the Avatar Project in XCOM2?

It seams inevitable that the Advent will complete the Avatar Project; am I supposed to be able to stop them, or is this just a count down timer. I mean the graphic is red so I assume something bad is going to happen.

Should I bother even trying to stop them or instead focus on research and building up my soldiers / base?

I haven't progressed enough to find out what the Avatar Project actually is, so please mark any spoilers accordingly.

Once all 12 pips are filled, a timer starts (based on the next time Advent would generate a pip of progress ~20 days). If you haven't reduced any Avatar progress by that time, the game ends in a loss.

Attacking Alien facilities will be your primary way to reduce Avatar progress, so be sure to acquire the resistance connections that make launching blacksite missions possible.