Combustion is when a material is rapidly oxidized by its surroundings. For example, wood can burn in the presence of the oxygen in air. For most combinations of materials, an oxidizer and what gets oxidized, extra energy from an outside source is required to break the chemical bonds. This is what you do when you put a lighted match on the wood - the heat from the flame can break the chemical bonds in the wood.
For a few combinations of materials, there is no extra energy needed to break the chemical bonds, and you can have spontaneous combustion. Thankfully, for humans, there is no naturally-occurring chemical under which parts of the human body would be able to spontaneously combust. You can burn humans, but it takes an outside energy source - they won't spontaneously combust.