Imagining a wave of water with people is the easiest way to explain visually what a wave looks like in a group of people. Using this image and starting at one end, the leader on the floor introduces the idea and gets everyone focused to stand up and raise their hands when it is their turn.
Starting usually at the far left of a bleacher or stadium the wave starts by the beginning rows standing and cheering and throwing their arms in the air, immediately sitting down afterwards.
As the wave progresses and the further sections see the section before them stand, raise their arms and sit down, they subsequently stand and raise their arms as the previous section is on it's way to sitting down.
If this pattern follows all the way across an arena it gives the impression of a human wave and is fun and can be a great encouragement for pep rallies, sports teams, or any of their kind of celebrations.
If the wave reaches to the end of a section often times the pattern goes backwards and the last people that were standing and cheering start the human back the other way. Don't be a party-pooper and stand and be counted in the human wave.