When there is a price floor, the price is higher than it should be. Sellers try to sell more of their product because the price is high. But buyers do not want to buy at that price. An example of this might be the minimum wage. At that wage, more workers might want to work (because the wage is higher than it would naturally be) but employers do not want to hire them because the wage is too high.
When there is a price ceiling, the reverse happens. The price is lower than it would naturally be so buyers want a lot of the product. But sellers can't make as much money so they don't want to sell. This can make it harder for buyers to get any of the product. An example of this recently was the cap on gasoline prices imposed by the state of Hawai'i in the mid-2000s.