Slow things don't move fast, but take a while to get where they're going or do what they're doing. Traveling on a slow train might be fun, but you'll get to California faster on a plane.
If it takes you a long time to finish a book, you're a slow reader, and if it takes years to become fluent in a foreign language, you can call it a slow process. Slow music is relaxing, with a leisurely beat. Sometimes slow means "boring or dull," like a slow movie plot, or "running behind," like the slow clock on the wall in your math class. As a verb, slow means "to reduce speed or intensity."