Unlimited Darts, also known as Unlimited Darts Mode, is a feature in Bloons and Bloons 2 that gives the player an infinite amount of darts. This can be optionally chosen on the menu at the bottom of the level. It helps a lot on some levels, but it is totally useless on others. The level will be marked as completed once the pop target is reached, but not once all on-screen bloons are popped.
In Bloons 2, at the end, instead of showing a score, it says "No Pain, No Gain..." which means to get points you have to be in limited darts mode. This is similar to when the player uses a Super Monkey. The player can still unlock additional levels, but occasionally there will be a glitch that prevents the them from unlocking certain zones.