Reviewing
"1" Answer, "Made a mistake"
This answer works differently to the others and the behaviour isn't obvious.
It reduces the interval by two 'steps', so if you have a card that goes 8 -> 20 -> 50 -> 120, '1' @ 50 days will bring it back to 8-12, and '1' @ 120 days will bring it back to about 20 days.
It doesn't tell you what the interval is, because there is a mandatory review period first. Also you shouldn't be worrying too much about the intervals when you fail a card. Let Anki take care of that and just worry about how far into the future the card will be sent if you mark it correct.
The mandatory review period: when you fail a card, it is shown 10 minutes or 8 hours later, depending on if the card is young or mature. When you review it again in that time, if you get it correct, the card will be scheduled back into the future. The review intervals are static (you can change them in the deck preferences). The reason it works this way is to ensure that cards you fail are not put back into the future without you being able to remember them. You have to prove that you can remember the card 10 minutes later or 8 hours later before the card will be scheduled back into the future.