What is this [i] in my shell?
FISH SHELL
This is probably something very obvious but I literally cannot work out what it is. I want to get rid of it.
What is the capital I in square brackets doing?
You seem to have enabled vi-mode, which triggered the mode indicator.
The "[I]" stands for "insert mode". If you press escape, you'll see it change to an "[N]" for "normal mode". If that doesn't make any sense to you, disable vi-mode again with set fish_key_bindings fish_default_key_bindings (you can get back to insert mode by pressing "i").
If you wish to disable this indicator, define an empty function called fish_mode_prompt.
If you didn't enable this manually, it might have been enabled via some third-party plugin or theme. I know for a fact the "budspencer-theme" for some reason "requires" vi-mode.