Generally you should leave this blank and let mojoportal handle the redirecting after a user signs in.
However if you want to force redirecting to some specific page you can force it with this setting:
<add key="PageToRedirectToAfterSignIn" value="/yourcustompage.aspx" />
Warning! Do not use this in a multi site installation. This is a global setting and will affect all sites.
If you change this from the default behavior it may cause inconvenience for your users.
Suppose a user is deciding to buy a download product in your store, they need to login to complete the purchase
currently the login will redirect back to the store page or whatever page the user was on and this is the desired action
so if you force the redirect somewhere else you break this.
Same thing if the user wants to post in the forums but needs to login, the default behavior will
redirect him back to the forum thread, if you change it you break this.
<add key="PageToRedirectToAfterSignIn" value=""/>
You could put that setting in user.config and set the value to /Default.aspx, but I recommend against it for the reasons shown in comments.