This first line "#!/bin/bash" is also called the shebang and tells the OS which program to use to interpret the script. after "#!" follows the path to the program

/bin/sh is the standard sh(ell) in linux, bash stands for the more advandced bourne again shell. both are shells and there are lots of other shells (ash, zsh, csh....) but they have to be installed to be used (of course...)

so he changed the program that interprets your install scripts from bash (that was not installed) to the standard shell.