To deflate is to let the air out of something. If you deflate the tires on your brother's bike, he won't be able to ride it until he gets them pumped up again.
You can deflate anything that's full of air or another gas: an air mattress, an inflatable sled, a helium balloon, or the tires on your car. When something, like a hot air balloon, empties of air, you can also say it deflates. Figuratively, a person can also deflate when they are suddenly drained of self-assurance or cheer: "Hearing him criticize me in front of the class made me deflate."