If there is a dearth of something, there is not enough of it. A dearth of cupcakes is unfortunate, but a dearth of nutritious food is a serious problem.
Dearth is an Old English noun formed from the adjective deore, "precious, costly," and the noun-forming suffix -th. Though the relationship of dearth meaning "lack, insufficient amount" to the adjective dear is not so obvious, it is still easy to imagine that something precious is probably also in short supply. Dearth is used almost exclusively in the phrase "a dearth of."