Ambient occlusion is darkening parts of a model which can see less of a sphere of ambient light surrounding them.
In other words, any concave part of a model will be able to see less than a full hemisphere of ambient light hitting it. So you figure out how much they can't see and darken those portions of a model appropriately.
If your model doesn't animate, then you could get this effect just by applying a lightmap to your model or even painting the texture directly.
If it does animate, then the shape of the surfaces will change and the lighting will need to change accordingly.
OJay's link there is Screen Space Ambient Occlusion. That's different. That's calculating ambient occlusion just using the z-values of the pixels on the screen. It's less accurate, and prone to artifacts like the background glowing around the tank turret you see in that image. But it's a lot faster.
In other words, any concave part of a model will be able to see less than a full hemisphere of ambient light hitting it. So you figure out how much they can't see and darken those portions of a model appropriately.
If your model doesn't animate, then you could get this effect just by applying a lightmap to your model or even painting the texture directly.
If it does animate, then the shape of the surfaces will change and the lighting will need to change accordingly.
OJay's link there is Screen Space Ambient Occlusion. That's different. That's calculating ambient occlusion just using the z-values of the pixels on the screen. It's less accurate, and prone to artifacts like the background glowing around the tank turret you see in that image. But it's a lot faster.