$? means that the return code of the previous command. When you write a script, we need to ensure that the command has successful or not. This will be done by checking the return code of the previous command. i.e $?

Upon successful execution of command, it will return the code 0 else non-negative no.
For ex:

$ rm tmp
$ echo $?
0

$ ls tmp
ls: tmp: No such file or directory
$ rm tmp
rm: cannot remove `tmp': No such file or directory
$ echo $?
1