Aliasing is what happens when you try to capture something the system can't quite resolve. In video, that means details too fine for the pixel grid to represent — a striped shirt at certain distances, a chain-link fence, a brick wall in a wide shot, a rotating helicopter blade. Instead of being captured cleanly, those fine patterns produce strange visual artefacts: shimmering, moiré patterns, "wagon-wheel" effects where wheels appear to spin backwards, jagged stair-stepped edges on near-horizontal lines.
The cause is geometric, not technical. Every digital system samples reality at discrete points — pixels in space, frames in time. When the thing being sampled changes faster than the sampling rate can keep up with, the system "guesses wrong" and produces a different pattern that wasn't really there. This is the same phenomenon as that famous aviation video where helicopter blades look frozen or stationary: the camera samples at 60 fps, the blades spin at a rate that's a near-multiple of 60, and an illusion is born.
In practice, aliasing matters because it's hard to fix once it's recorded. The standard prevention is good optical low-pass filtering at the camera and proper anti-aliasing in computer-generated imagery — both make sure that details too fine to capture get gently blurred away before sampling. For products that handle user-generated content, aliasing is a constant background issue: a 1080p stream of a textured shirt or a brick wall will look noisier than a 1080p stream of a face, because the small details cause more aliasing. Higher resolution and good downscaling filters help, but the root fix is at capture.
