What should you focus when proofreading

Spelling, grammar or punctuation.

One of the most important things that you should focus on whilst proofreading a biography or essay is conventions.

I do not mean the place "conventions", another meaning of convention would be: correct spelling, punctuation, capitalization, and grammar in writing terms.

So when you, or the audience could read your biography or essay, they do not have to pause or linger on your "typo" or spelling/grammatical mistake, instead read your passage fluently.