Sedimentary layers are laid down on top of one another. These layers are normally horizontal and at the same angle. An uplift can shift the angle of the horizontal layers but the layers are still at the same angle.
A period of erosion can create a new horizontal horizon. A new sedimentary layer will be horizontally laid down on shifted layer below creating an unconformity.
Sometimes horizontal layers are considered to be unconformities if the fossils found in the layers do not follow the sequence proposed by the Darwinian theory of evolution. Even though there is no physical evidence of erosion the layers are considered to be an unconformity based on the fossil evidence.