It's really hard, but in essence, nerves from the brain go up into the nose and on these nerves exposed in the nose to the outside air are receptors, and odor molecules coming in on a sniff interact with these receptors, and if they bond, they send the nerve a signal which goes back into the brain.
Doista je teško, no, u biti, živci iz mozga pružaju se do nosa i na tim živcima u nosu izloženima vanjskom zraku nalaze se receptori, a dolazeće mirisne molekule prilikom udisaja uzajamno djeluju s tim receptorima, te ako se povežu, šalju živčani signal koji se vraća nazad do mozga.