In a nutshell, proofreading means reviewing a text for errors in spelling, punctuation and grammar. But it is more than just making a quick check for correctness. Usually it is the final step in the writing process and polishes your work.
It used to be most common to print the text but nowadays better screens make it possible to identify all the little errors in the document without having to print it.
The main part of proofreading is the word-by-word check without distraction from the content – including contextual mistakes which automatic spell-checking programs often won’t catch.
The proofreader’s best friend is the dictionary. S(he) requires this book or program to verify the meaning, derivation or grammatical use of words.
Blunders in the content structure will be recognised and noted during the proofreading stage, but for smoothing and polishing you’ll also need a copyeditor.