Subcutaneous tissue, also called the hypodermis or subcutaneous layer, is the layer that sits just below the dermis of the skin. According to OpenStax Anatomy and Physiology, its main job is to connect the skin to the fibrous tissue covering the underlying muscles and bones.
It is made of loose connective tissue and fat-storing adipose tissue and has a good blood supply. That fat serves as an energy store and also insulates and cushions the body. Strictly speaking it is not part of the skin itself, although the boundary between it and the dermis can be hard to see.
Source: OpenStax Anatomy and Physiology 2e: 5.1 Layers of the Skin
Image: cross-section of skin layers (the subcutaneous layer is marked C), U.S. government, public domain, via Wikimedia Commons.