It’s called a clef.

In the staff, if # is on that line or space, the note will rise. At the same time, after the treble clef, it also indicates the key signature. For example: 1 # is G major or E minor, 2 # is D major or B minor, 3 # is A major or F sharp minor, 4 # is E major or C sharp minor, 5 # is B major or G sharp minor, 6 # is F sharp major or D sharp minor. Extended information

When using staff notation, it is also necessary to add notes and rests indicating different durations; clefs to determine the pitch position; key signatures and other symbols to indicate the key. The predecessor of the staff is the "newm" symbol that appeared in European church music in the 7th century AD. It was later developed into four-line notation by the Italian music theorist Guido.

In the 13th century, Cologne priest Franco and others invented the quantitative notation method, which supplemented and improved the "Newm" symbol. After centuries of improvements and improvements, the staff began to take shape in the 18th century and is still in use today. Guido is considered one of the founders of the staff.