The heat that is present in Earth's mantle is made up of leftover heat from when the planet was formed, latent heat from the Earth's inner core and heat produced by the decay of radioactive isotopes. The majority of the heat is caused by the decay of isotopes such as potassium 40 and uranium 238.
When an isotope breaks down, it sheds extra energy so that it can become more stable. This extra energy is radiated as heat and warms the mantle.
Earth's core contains liquid metal that expands a little bit every year. This expansion causes heat to spread to the mantle.
The Earth was created when gravity caused particles and hot gases to condense into a planet. Heat was also created by this process, and some of the heat is still around, warming the mantle.