The left prompt is ordinarly a single $ character, however other characters are displayed in different vi modes and within Nix shells. It's prefixed with a red ! upon the previous command's failure, and shows the last backgrounded process, if any.
The right prompt is your PWD in short form ($HOME/abc/def/ghi is shortened to ~/a/d/ghi). If you're in a Jujutsu or Git repo further contextual information will be displayed.
Install with your favourite Fish package manager, for example:
$ fisher install samhh/fish-minimal-theme