To edit a piece of writing means to improve it. This can involve correcting mistakes and replacing weak words with stronger ones.
The hot yellow sun was shining in the sky as the tired camels went to the nice watering hole.
There are several weak words in this sentence that could be improved. We know that the sun is hot and yellow, so it doesn't really need to be mentioned, and 'nice' is always best avoided!
The sentence could be re-written something like this:
The sun blazed out of a cloudless sky as the weary camels trudged towards the welcome watering hole.
Editing a text can also involve removing ambiguity. An ambiguous sentence is a sentence that doesn't make the meaning clear.
Large table required for artist with folding legs.
It would be better to write:
Large table with folding legs required for an artist.