This can be set to Ultra if your GPU is always being fully utilized (99-100% usage while playing a game).
Low Latency Mode (Anti-Lag on AMD) controls how many frames are pre-rendered in the buffer and is meant to reduce input lag. This can be set to Performance, which means it will apply AO quickly when it is needed instead of maximizing fidelity.
Ambient occlusion is a shading and rendering technique used to calculate how exposed each point in a scene is to ambient lighting.
