When I learned to play piano as a kid, there were the white keys and the black keys. The black keys were the sharps and flats. If the key signature told you about any sharps or flats, then you automatically played those every time you saw that note. Or, if the note wasn't always sharp or flat, just sometimes, then it would tell you at the time the note turned up.
Now I'm learning to play the ukulele. What the ukulele shows is that all the notes proceed equally one after another. The piano is designed in the key of C. On the piano, F is a white key, but you know, if you are playing in the key of D, then F actually becomes a black key, a sharp or flat.
It's so cool when you see things as fitting into one framework, and then your mind expands and you can see things in a different way.