It is a convolutional neural network (CNN) that performs image segmentation. This means that the network learns to assign each pixel a class depending on the object or surface it belongs, e.g a car, highway, tree, building...

It uses an Encoder-Decoder architecture, were the image is first downsampled by an encoder as in a "traditional" CNN like VGG, and then it is upsampled by using a decoder that is like a reversed CNN, with upsampling layers instead of pooling layers.

The network is explained in detail here: A Deep Convolutional Encoder-Decoder Architecture for Image Segmentation