The term level is mostly used in tournaments. A tournament is divided into levels. Depending on the structure, the tournament can have 20, 50 or 100 levels. The tournament starts at level 1. It is stated that level 1 will last for 15 minutes. During this time the blinds will be 25/50 with no ante. After 15 minutes of play, we will move to Level 2. Level 2 will have bigger blinds. With each new level in the tournament, the blinds will increase. So for Level 2, let's say the blinds are now 50/100. And so on until there is only one player left in the tournament.
Always be on the lookout for tournament levels
It is important to keep levels in mind when playing tournaments because depending on the level, it can alter your playing style. Let's take a concrete example to understand this. You have 15 big blinds in a tournament in cutoff position. You receive a medium-sized starting hand. You are hesitant to open it. With 15 big blinds, it's hard to raise-folder so you think about pushing all in, but you're not sure. If the next level is coming up soon and the raise will give you 8 big blinds instead of 15, it might encourage you to play your hand.