Above mean sea level means how high something is above the average level of the ocean. We use the ocean as a reference because it makes it easy for everyone to understand how high something is.
It's kind of like when we play with blocks. We stack them up and we use the bottom block as our starting point. If we say the top block is 4 blocks high, that means it's 4 blocks above the bottom block.
In the same way, when we say something is 100 meters above mean sea level, that means it's 100 meters higher than the average level of the ocean. It helps us understand how high something is, and it's important for things like building houses and roads, planning cities and understanding weather patterns.