Proofreading means reading a proof copy of a text in order to detect and correct any errors.
Proofreading in printing and publishing
A proof copy is traditionally a version of a manuscript that has been typeset after copyediting, but the line between copyediting and proofreading is thin. When handwritten originals were common, it was often easier for a copy editor to review and mark up a manuscript after it had been typeset. With computer typesetting from electronic manuscripts, the distinction becomes even more blurred.
Proof typescripts often contain typographical errors introduced by mistyping (hence the word typo to refer to misplaced or incorrect characters). Traditionally, a proofreader checks the typeset copy and marks any errors using standard proof correction marks (such as those specified in the international standard ISO-5776 or more specifically for English, the British StandardBS-5261:2). The proof is then returned to the typesetter for correction, and in many cases the production of a second proof copy (or "revise").