Z-fighting is a modelling glitch that occurs as a result of two or more faces that either overlap or share similar coordinate values. This appears as flickering textures both in-game and in 3D modelling software.
How to Fix it
The only way to stop faces from fighting is to move them farther apart from each other. Depending on the viewing distance, farther viewing distances will require more space between faces to avoid fighting while short viewing distances will only require anywhere from around 50 units at a minimum.