In order to become a more proficient writer, you need to be able to think of your writing in "global" and "local" ways. According to the Allyn & Bacon Guide to Writing Concise Edition, "You revise locally whenever you make changes to a text that affect only the one or two sentences that you are currently working on. In contrast, you revise globally when a change in one part of your draft drives changes in other parts of the draft" (275).
Basically, global revision involves the big picture of your essay; it relates to ideas, purpose, audience, evidence, analysis, and organization.
Local revision focuses more on sentence-level revision: changing words so that a sentence is clearer, correcting grammatical or spelling errors, etc.
As a writer, part of your job is to be a successful global reviser as well as a successful local reviser.