Econ - Deadweight loss - over & under production

Can anyone explain to me how do this deadweight loss happen?

In simple terms, when producers are forced to produce less than what they are willing to produce (say because of a quota system, or as a result of a tax on the market), and when consumers are not able to buy all they are willing to buy, the result is a market in disequilibrium. The deadwight loss is basically all the quantities that the consumer wishes to buy, but can’t, multiplied by the price of each item of that quantity. I 'm trying to state it differently to get another view on it.