Stars are big balls of mostly hydrogen and helium, the two most abundant elements in the UNIVERSE. Now because stars are so massive, roughly 1,000,000,000,000,000,000,000,000,000,000,000 time more massive than you are, the PRESSURE at the center of the star is very very compressed... and when a gas gets compressed it heats up because the atoms are closer together and potential energy is smaller, but the kinetic energy, a direct measure of temperature is higher. The temperature in the core of the sun is about 15 million degrees!!! Pretty hot!

Now, at these high temperatures, then two atoms of hydrogen slam into each other, the protons stick and a new element is made . . . it is called helium. In that NUCLEAR process a huge amount of energy is generated and the energy is carried to us as sunlight! This is the same reaction that powers a nuclear hydrogen bomb. The conversion of hydrogen H to helium He, plus lots of energy.

Stars are basically giant hydrogen bombs: the interior gets hot enough and under enough pressure to fuse hydrogen into helium. The only reason that the explosion doesn't blow them apart is because they have enough gravity to prevent that from happening.