Anaglyph 3D renders your scene twice from two slightly different camera positions — one for each eye.
The RED channel carries the left-eye image. The CYAN channel carries the right-eye image.
Your glasses filter these so each eye sees only its own image. Your brain fuses them into depth — making objects pop out of and sink behind the screen.
The Stereo Depth slider changes the distance between the two virtual eyes. More separation = more dramatic (and more eye-straining!) 3D.
Tip: red lens on the LEFT eye. If depth looks inverted, flip your glasses.