Smell that cake baking? No? Take a sniff. A sniff is when you take air in through your nose to smell something.
Dogs sniff a lot. It's how they get to know the world. Imagine this when you see the figurative use for sniff meaning to snoop as in, "The detective was sniffing around for clues." Sniff is one of those words that sound like what they mean. Take a big sniff and listen, you'll see. If you have a cold, and you keep sniffing, you have what's known as the sniffles.