By default, it will send to the email address you have in your site Configuration (in the User tab). You can specify it in the snippet tags with the parameter &to=`
[email protected]`.
This can be a comma-separated list of addresses; it can also be a value from the form, such as from a list of optional departments to send a message to; &to=`[+department+]`
If you really want to get fancy, you can use a custom snippet/function and the eFormOnBeforeMailSend event and set the $fields[’to’] value in the function. I prefer to use an external config.php file for eForm’s configuration if things are that complicated, though. Since it’s a normal .php file any complex conditionals can be done there, without the added overhead of more snippets.
(I know, more information than you wanted, but hey! This is MODx; there’s always more than just one or two ways to do things!)