I expand knowledge on topics people don't pay attention to, especially among geographic locations, ethnic groups, languages and world history. Otherwise, I build article structure or integrate articles more into Wikipedia and clean up grammatical errors, missing hyperlinks, wording, ambiguity or comprehensibility, neutral point of view and layout, add redirects or other. I also may create articles, typically from red links but sometimes not, in order to add to an area or category of low coverage.
I normally come across the pages I edit in my everyday research. These often include articles which were last edited a long time ago, stubs, articles without sources and other hard-to-find articles.