https://unix.stackexchange.com/questions/23111/what-is-the-eval-command-in-bash

bash man page:
eval [arg ...]
    The  args  are read and concatenated together into a single com-
    mand.  This command is then read and executed by the shell,  and
    its  exit status is returned as the value of eval.  If there are
    no args, or only null arguments, eval returns 0.

This is a common function in many languages, e.g. Perl and JavaScript. Have a 
look at perldoc eval for more examples: http://perldoc.perl.org/functions/eval.html

Last but not least, eval can be a very dangerous command. Any input to an eval 
command must be carefully checked to avoid security problems.