Why is my RAM nearly fully used?

There's more memory available there than you think. "Inactive" memory is memory that has been used, and is essentially keeping its old data so that it can be swiftly grabbed again if needs be. However, if your computer needs the memory for some new process, that memory is capable of being used - the old data is dropped out, and new data is put in.

As others have said, you also need to show all processes for a full idea of what's going on with your memory. But you're not nearly as tapped out as it first appears.

You are displaying only your processes 'Mes opérations'. You can switch to 'All processes' to see what is using the remaining part of the memory.