It is Fuel: Just like a car needs gas to move, a character needs mana to use special powers or spells.

2.It is a Limit: You only have a certain amount. This stops you from using your strongest moves over and over again, which keeps the game fair and challenging.

3.It Runs Out and Refills: When you use a spell, your "tank" goes down. To get more, you usually have to wait for it to refill slowly, drink a potion, or rest.

4.It Requires Strategy: Because you can run out, you have to decide: "Should I use a small spell now, or save my mana for a big attack later?"
In short: Mana is a way to limit how much magic you can use so that you have to think carefully about your choices.