This is the exit status of the last executed command.
For example the command true always returns a status of 0 and false always returns a status of 1:
true
echo $? # echoes 0
false
echo $? # echoes 1
From the manual: (acessible by calling man bash in your shell)
$?Expands to the exit status of the most recently executed foreground pipeline.
By convention an exit status of 0 means success, and non-zero return status means failure. Learn more about exit statuses on wikipedia.
There are other special variables like this, as you can see on this online manual: https://www.gnu.org/s/bash/manual/bash.html#Special-Parameters