Burnt umber is a reddish-brown. You already understand that when you see it. If you heat it, the color becomes more intense. If you don't, nothing will change.
Burnt umber is a cool blend of yellow and red, with a hint of gray mixed in. It is one of the three primary colors of paint and can be sometimes confused with sienna or umber. It is more complex than any other color on the color wheel thus giving it an advantage for more useful applications.
It is a brownish color with a strong, brownish red undertone. It is the brownest of all brown paints, and gets its name from being burnt to darken it.
This color was borrowed from the old brown, but it is much darker and warmer. It was added to the same triad of colors in a more diluted form, but adding black to brown makes a lot of difference. Similar to how a glass of cold water looks more transparent than a glass of warm water.