The explanations on how to fix this are probably correct, but here's intuition for WHY it's happening. Likely, you have accidentally duplicated your original object, but left the duplicate in the exact same location as the original. This means that the faces are "fighting" with each other for visibility (as faces are infinitesimally thin, only one face can prevail. However Blender doesn't know which face prevails) and so that's where you see the artifacts.
I believe this is referred to as "z-fighting".
Deleting the duplicated object will probably fix your issue.