The reference contains a list of three things:
ELK ships with a number of layout algorithms you can use. To use one, you will need its identifier. To configure it to your use case, you will need to know which layout options it supports and what they do. This part of the reference has all of these information.
Layout options are used to configure layout algorithms. This part of the reference lists them all and describes what they do. You will probably want to go and check what you layout algorithm of choice supports first.
Layout option groups organize layout options. This part of the reference is probably only of interest to few people.