Umami is a fundamental taste just like salty, bitter, and sweet. The word umami is a Japanese word, and in English we often describe umami as a savory-sweet taste.
Tea leaves will naturally produce components that give the umami taste in the cup. Mushrooms, tomatoes, meat, and seaweed, for example, contain a lot of umami. You might experience a soup-like, meaty, savory, or even marine and seaweed-like character when you drink a tea like gyokuro, sencha, or matcha. This is the essence of the umami taste!