ResNet is a convolutional neural network architecture designed to make very deep networks trainable. Simply stacking layers can produce the degradation problem: deeper networks may have higher training and test error than shallower ones, even without vanishing or exploding gradients. section 11.2.3
A deeper network should be able to reproduce a shallower network by learning the identity function in its added layers, but ordinary layers struggle to learn identity mappings. ResNet adds shortcut connections so layers can learn a residual transformation instead: if h(x) is the desired mapping, the stack learns g(x) = h(x) − x while the shortcut carries x directly. section 11.2.3 section 11.5
Why the shortcut matters
On CIFAR-10, a 56-layer network can have higher training and test error than a 20-layer network. The residual design addresses this counterintuitive degradation by making the identity behavior available through a shortcut rather than forcing every added layer to learn it. section 11.2.3



