The idea is to extend the staff in whichever direction. Back at one time, music was actually written with about 15 staff lines, but it was very hard to read all those lines just close together. So, they came up with the idea of using a pair (for a grand staff, like piano music) of five line staves, with space between for a single "ledger line" for middle C.
Notes higher than the treble staff are rendered on ledger lines, as well as notes lower than the bass staff. Think of it as a little ladder sitting on top of the staff.
The concept is not perfect, it is hard to sight read music that has many ledger lines (like when the ladder gets more than 3 or 4 steps). For that reason, other signs are also used (like 8va or 15 va - meaning to play an octave or two octaves higher - 8vb or 15vb - meaning to play an octave or two octaves lower).
Just another part of the way that music is represented on paper.
Jim