Deadweight loss is an economic loss to the public without any offsetting gain.
The loss represents the extra value that consumers obtain goods and services that is worth more to them than the price they paid. For example, you may buy lunch for $10 but you may be so hungry that it is really worth $15 to you. If the price of that lunch were raised by a monopoly to $16, then you wouldn't buy the lunch and your individual "deadweight loss" from that monopoly price would be $5 compared to the free market.
In other words, the deadweight loss is due to the loss in value to society of output not produced.