@Worik we don’t have an index of terms, we have the documentation site. Everyone contributes to make it better, maybe you will be the one that builds an index. 
If you need help with specific code, ask the person that wrote it. If it’s in a theme, ask in their issue tracker. Or, open a new support ticket after reading Requesting Help. Hugo is very complex, and I recommend you read every page in the docs, to start.