Rebirth trades your current Damage stat and levels for permanent multipliers on your Damage gain rate and Cash earnings. You keep your Cash, upgrades, and TNT unlocks. Rebirth when your Damage growth stalls, then rebuild through the training area.
Rebirth is the game's permanent-stat reset: everything you trade is listed below, with what is confirmed and what is still player-reported.
For +1 tnt mining rebirth, the short version: reset when your Damage growth stalls, keep your cash and unlocks, and rebuild through the training area so every cycle starts faster than the last.