Sometimes you understand it immediately.
Sometimes you have to think for a moment or reread something.
Then you start playing the game and notice how everything comes together. You use what you have read immediately.
While playing, you encounter moments where you get stuck. A rule that turns out to be trickier than expected, or something you don't quite know yet.
If you want to progress in the game's adventure, you will sometimes have to learn something new.
Not everything at once, but exactly what you need at that moment.
After that, you continue on your way.
And that is how you notice that you understand what you read, what you do, and how the game works better and better. Without it feeling like just learning.