The Circle of 4ths is one of the most revolutionary tools used in music for many purposes and understanding musical concepts at a glance.
The Circle of 4ths is a circle consisting of keys in order of intervals of perfect 4ths.
C-F-Bb-Eb-Ab-Db-Gb(F#)-B-E-A-D-G-C
if you observe it rotates in a clockwise direction in such a way that it returns to C and starts over.
Now if you watch closely you can see the key signatures interior of the circle.
This helps you remember your key signatures easily.
This means F is 1 flat, Key of Bb is 2 flats, key of Eb is 3 flats, etc.
So anytime you lose touch with the key signatures, just refer to the Circle of 4ths and you will see how easy it will be for you.
Finally, also take note that the circle of 4th is also referred to as the Circle of 5ths when it goes in the anticlockwise direction.
More revelation about how useful the circle is will unfold in the future.