The core problem they solve
Every 3D method needs to route a different image to each eye. Glasses do that by filtering light with color, polarization or shutter timing. Autostereoscopic displays do it optically, at the screen itself, so no eyewear is needed.
Lenticular lens arrays
A sheet of tiny cylindrical lenses sits over the pixel grid. Each lens directs light from specific sub-pixel columns toward specific angles in space — one set of columns reaches your left eye, another reaches your right, purely based on your viewing angle.
Parallax barriers
An alternative approach places a precisely striped opaque barrier just in front of the pixel layer, physically blocking each eye from seeing the columns meant for the other. It's simpler to manufacture but sacrifices some brightness and resolution.
Why they need eye or head tracking
Both methods only work within a narrow 'sweet spot' of viewing angles. Modern displays like Looking Glass panels add real-time eye tracking to shift the effective viewing zone as you move, keeping the 3D effect locked on instead of collapsing when you lean sideways.
The trade-off
Glasses-free 3D trades some resolution and brightness — since the panel is doing double duty splitting light per eye — for the comfort of not wearing anything. That trade-off is shrinking fast as panel resolutions climb.
See it in action at 3dstreaming.org