If two different things coexist, they are both there at the same time, or in the same place. Two ideas can coexist in one person's mind. Two groups can coexist in one city. Two kinds of animal can coexist in one forest. The word does not, by itself, say that they like each other — only that they are both present. Though actually, people often hope for a softer meaning: they say coexist when they mean "live side by side without fighting." You will hear "learn to coexist" in talk about neighbours, religions, or political camps. In science it can be quite dry: two chemicals coexist in a mixture; no friendship is implied. It is a bit formal, but not rare. Students meet it in textbooks and news.