The "Salish Sea" is the name collectively given to three related bodies of water: The Puget Sound, The Strait of Juan de Fuca, and the Strait of Georgia. These three seas form, respectively, the inlet between The Olympic Peninsula and the rest of Washington, the border between the Olympic Peninsula and Vancouver Island, and the strait between Vancouver Island and the mainland of British Columbia. Although they are separate bodies of water, they are related in terms of hydrography, surrounding terrain, and the biome and traditional cultures around them. The name "Salish" comes from the Salish Peoples, who were the indigenous inhabitants of most of the area before the coming of Europeans.
The main benefit of the term "Salish Sea" is it is a convenient way to refer to a natural geographical area. Even I, a native Washingtonian, would have inaccurately said, for example, that Bellingham or Port Angeles were "on the Puget Sound", which is incorrect. However, they are both on The Salish Sea.