Why does the graph go up and down?

May seem extremely obvious but this is what I’ve gotten from a source:

“If many people buy and there are not enough sellers who would sell the asset to buyers, the price goes up because of high demand. Vice versa, if there is not enough demand and people are selling, the price goes down. “

The problem here is that the claim that’s being made is that if many people buy and there’s not enough sellers the price will go up because there’s a higher demand or course. But how can people “buy” if there’s not enough sellers? Why doesn’t it say “If many people place a buy order…” or “If many people TRY to buy…”. If you “BUY” something it means that the transaction has gone through and that there are available sellers. This is giving me a headache.