There are many concepts, structures and systems in music theory, some remedial, some not so... This one's a bit mental.
Fear not! I’m here to teach you a lesson (I promise).
The circle of fifths provides a fundamental basis for harmony, key, scales and general theoretical application.
There are twelve different notes in Western music, with them, we form a “key” diagram in a circle; think of a letter-coded clock face. (Yeah, it’s weird)
Now we’ve got a rough picture...
The keys go up in fifths clockwise and down a fifth each time anti-clockwise starting with “C” at noon. Each key is also coupled with a “relative minor key” (as if it wasn’t hard enough) which follows accordingly honing the same flats or sharps as it’s major counterpart.
As you go clockwise you gain a sharp in your key signature, anti-clockwise from noon provides a “flat” addition each key.
You can have up to six sharps or flats in your key signature with “C Major/A Minor” having none of either and “F# Major/Gb Major - D# Minor/Eb Minor” having six relatively - crazy right...?
Understanding “key” is key (get it?) in terms of getting to grips with music.
If any of that has sunk in here’s the nitty-gritty.
So there we have it a small, albeit, jam-packed blast of information in a nutshell, please digest safely and responsibly!