The bind builtin causes fish to add the readline style bindings specified by BINDINGS to the list of key bindings, as if they appeared in your ~/.fish_inputrc file.
For more information on the syntax keyboard bindings, use man readline to access the readline documentation. The availiable commands are listed in the Command Line Editor section of the fish manual - but you may also use any fish command! To write such commands, see the commandline builtin. It's good practice to put the code into a function -b and bind to the function name.
bind ''\M-j': jobs' Binds the jobs command to the Alt-j keyboard shortcut