This is the default built-in rendering engine, which is also used by OVITO to render the contents of the interactive viewports. Since it is hardware-accelerated, it is very fast. Unlike the Tachyon renderer, the OpenGL rendering engine does not support the display of semi-transparent objects at the moment.
To reduce aliasing effects, the output image is usually rendered at a higher resolution than the final image (supersampling). This factor controls how much larger this resolution is. A factor of 1 turns anti-aliasing off. Higher values lead to better quality.