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 choosen 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 target is reached, not once all 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 player from unlocking certain zones.