Key signature is the number of sharps and flats a key has for us to easily identify it. When you look at a musical piece, you would find out that at the very beginning of the piece you would see series or a single sharp(s) or flat(s). They are there for you to determine the key the piece is written on. Look at an example in the picture above.
Those two sharps there tells whoever wants to interprete the piece the key of the song, so how does the circle of fifths and fourths help us to do this:
How do you determine the key signature of a song using thecircle of 4ths and 5ths
The left hand side of the clock are the flats, while the right hand side are the sharps:
Therefore according to the diagram
G = #
D = ##
A = ###
E = ####
B = ##### or bbbbbbb
F# = ###### or bbbbbb
Db = ####### or bbbbb
Ab = bbbb
Eb = bbb
Bb = bb
F = b
C = natural
Lesson 1: Introduction to the Piano
Lesson 2: Brief Introduction to music
Lesson 3: Keys of the Piano
Lesson 4: Singing and playing on different keys
Lesson 5: Intervals
Lesson 6: Chromatic Scale
Lesson 7: Circle of Fifths and Fourths
Lesson 8: Key Signature
Lesson 10: Fingering for the chromatic scale
Lesson 11: Introduction to Chords
Lesson 13: Chord inversions