Moiré is a visual artifact that can appear on your footage when you shoot a pattern with very tight parallel lines.
These artifacts are a result of insufficient processing power leading the codec to skip lines as it compresses the footage.
Unfortunately you can’t trust the camera viewer when looking for moiré because the compression will only happen later. It is also a hard problem to fix in post-production. So it is important to predict where this problem might appear and keep those textures out of focus.