In the simplest sense, positive means good — or the opposite of negative. If you have a positive attitude about homework, for example, you're more likely to get positive feedback on your report card.
Keeping track of positive's many meanings can be positively confusing. For starters, if you're positive about something, you're totally sure. In math, any quantity greater than zero is positive. In medicine, a positive result on a test indicates the presence of a disease. And that isn't a very positive experience for anyone.