A yawn is where your mouth opens wide and you take a deep breath in and out. Yawns just happen by themselves, they are hard to stop.
People and animals yawn when they are tired, stressed, overworked or bored, or for no reason at all. Yawning is infectious - if someone near you yawns, you often start yawning too! You might even start to yawn by looking at a photo of someone yawning, so don't stare at the photo of the yawning cat for too long.