The Sun emits energy (light, heat, UV rays, etc.). To do that it uses up a fuel through a process.
The process is fusion (not fission) whereby four atoms of hydrogen (four protons) are "fused together" to produce one atom of helium (2 protons + 2 neutons), two positrons (also known as anti-electrons) and two neutrinos, plus some energy.
The energy comes from the fact that all this (helium + positrons + neutrino) has a bit less mass than four hydrogen atoms.
E = m c^2
The energy (E) comes from some of the mass (m) being changed into energy (the c is a constant to make the units come out).
The Sun uses up over four million tonnes per second. It cannot last forever.
The bigger problem is that the protons fuse because they collide (the core of the Sun is very hot and the pressure is extreme, it is therefore "easy" for protons to collide at great speed, and fuse) and these collisons are random. Each time an helium atom is produced, it just sits there and gets in the way.
In roughly 5,000,000,000 years (five billions) the Sun will still be 90% hydrogen, but there will be enough helium to interfere with the random collisions, and that will be enough to "clog the furnace".