SPOILER!!

I chose to evacuate, and i was wondering what happens if i choose to reset? and what is New Game Plus?

Everything goes back to the way it was before, and the same sequence of events plays out again. Essentially creating a never ending loop, which the game implies at one point that that is what has been going on for a while now. The only way to break it is to just evacuate.

New Game Plus is where you play through the game again, with all of your weapons and stats from your first playthrough.

If you're playing it on the Steam version, you'll get a nice surprise at the Arsenal once you get the Bastion back up and running. Look for a Cannister (sic)

@IBurningStar said:

Everything goes back to the way it was before, and the same sequence of events plays out again. Essentially creating a never ending loop, which the game implies at one point that that is what has been going on for a while now. The only way to break it is to just evacuate.

It implies it in like the very first line of the game...

You keep everything in new game plus and you can make it harder with the idols