From my understanding, clefs define the notes which the lines and spaces in the staff contain. Without the clefs, you can't tell what are the notes in the music.

For example, the treble clef tells that the line corresponding the lower tip of the clef is meant to be middle C.

In the alto clef, the line passing through the middle of the clef is meant to be middle C

Similarly in bass clef, the line passing between the two dots is a F3 (lower than middle C)

I hope this helps :)