If something is legible, that means it’s possible to read it. This word refers to the quality of handwriting or the words that have been printed on a piece of paper.
This note is barely legible. (It’s hard to read because the quality of the handwriting is bad.)
The words written by the three-year old boy were clearly legible.
You need to learn how to write more legibly. (The word "legibly" is an adverb.)
Doctors are known to write prescriptions that are not very legible.
Is this legible enough for you to read?
This is illegible. (illegible = the opposite of "legible.")