Deadweight loss is an economic concept that happens when an outcome is inefficient and not "fair" for the people involved. In other words, it happens when something isn't working the way it should and the people affected lose out, or don't get what is most fair or best for them.
For example, if you wanted to buy a shirt at a store and you offered to pay $10 for it, but the store charged you $25 instead, the store and you both lost out. You lost out because you could have paid $10, and the store lost out because they could have made more money if they charged you the $10. This example of the store charging more than it should is an example of deadweight loss.