Illustration of an aerial view of an endless swimming pool

Quick answer：

An endless swimming pool, also known as a swim spa, is a long pool that allows swimmers to exercise continuously without turning around. The illustration you're referring to would depict an overhead view of such a pool, often showing the streamlined design and the continuous flow of water that enables the endless swimming experience. These pools are typically used for exercise and relaxation and are designed with hydrotherapy benefits in mind.