Well the main factor that makes the AK47 and its more common update the AKM so reliable is the very loose tolerances in its design. What this means is that the gun is put together in a way that allows a fair amount of room between all the moving parts. The result is that dirt and grime and carbon buildup and other stuff getting all inside the gun don't actually cause jamming or locking up, as opposed to for example the M16. The M16 is a fine weapon, but requires fairly constant maintenance to keep working, because its tighter tolerances make it more susceptible to its getting dirty preventing its function.

For example, if you drop an M16 in mud, you gotta clear it out before firing. Otherwise, the moving parts will get clogged up and jam. With an AK, however, it will most likely still fire as there is enough space between its moving parts to account for the mud.

This comes at a cost in the form of reduced long range accuracy and less consistency in quality between individual firearms, which is itself an indicator of different firearm philosophies between the USSR and the USA militaries.