GI is Global illumination. It’s not substandard, but it does take longer to render and isn’t really suitable for animations. There are methods to simulate GI and I would highly recommend that as you learn lighting, you look into those.
There are a couple components to Carrara’s GI settings. Briefly, Skylight allows you to use HDRIs, Realistic Sky, colors, etc. in either the Atmosphere (Realistic Skies) or the Background (colors, image maps HDRI) in the Scene’s effects tab. Skylight will treat items in those categories as lighting models and generate light based on your image or atmosphere. Skylight does not to my knowledge calculate reflected light.
Indirect Light calculates reflected or bounced light, but not atmospheric or image based light models. Additionally, it also can use objects with a glow in the shader’s glow channel as a light source.
Photon settings and accuracy seem fairly scene dependent. I’ve used low accuracy and l,ow photon counts and had the image look fine, and then in another scene the same values look like crap. My rule of thumb has been to start low and go up, rather than crank everything to it’s maximum and take a week to render.
Another tip to using Skylight and Indirect light is to turn off the scene’s ambient light. It can make the final image look washed out. If you use any lights in your scene, you may need to lower the brightness for those as well.