What does No Limit mean in poker?

No Limit is a poker format in which a player can put in any amount of chips, up to the full stack, in a single action.

How does it affect the game?

This format creates more pressure and makes mistakes more expensive. In no-limit games, the cards are only part of the picture. Stack size, position, timing, and the threat of an all-in matter just as much. That is why No Limit is considered more aggressive and more dynamic than fixed-limit formats.