Proofreading written copy before it goes to print or publication is the process of checking for spelling, grammatical, typographic, punctuation error and text layout.
It might seem easy at first sight, but using your eyes as a scanner is not because your brain often ignores inconsistencies and typing errors. GDA recommend someone else proofread your work, or at least use a tool like Grammarly, as no matter how many times you go through the text, you will nearly always miss a detail.
As an agency, we come across errors where work signed off by the client went to print, only for a glaringly obvious typo to the reader, is missed when proofreading.
As is usual, it is the writer or client who has final responsibility for proofreading. As an agency, we work with skilled freelance proof-readers and translators whose job is to pick out errors before output and edit copy to best target a subject and be on-message.