Inflation
Inflation is the slow increase in prices for things over time. Meaning the money you have right now will buy you less in the future.
It’s like a game where the points you earn gradually buy less in the game's store. Imagine earning 100 points in a game, and with those points, you can buy a power-up. However, as the game updates (representing time passing in the real world), the same power-up starts costing more points, like 120 points instead of 100. This means your 100 points now have less buying power in the game.
Similarly, with inflation, the money you have buys less over time, so something that costs $10 today might cost $11 or $12 in the future.