There are two potential sources for information. First of all, if it’s a built in shortcut, it is normally in the help documentation. For instance, if you do :help CTRL-I, it’ll take you to a help entry about moving around your jump list.
It is possible, however, that you have a custom mapping from one of your .vimrc files or an underdocumented plugin. In that case, try using :map which will list all custom keyboard mappings that are currently active. That will give you three columns:
- the mode the mapping applies to
- the keyboard shortcut
- the command that is run
You can then use :help to further investigate the command that is run.