The follow-up selection affects when you get to call back the lead and how you'll be notified about it.
Selecting after will simply postpone the lead and put it back in the top of the queue at the time of follow-up.
Selecting in exactly will schedule the lead and prioritize it in the top of the queue when it is due, and send you an email notification* within a couple of minutes after it becomes due if you are not already looking at it. Leads scheduled for an exact followup is listed under the "Scheduled" submenu.
* In order to receive email notifications about due leads, you need to enable those in your notification preferences (enabled by default).
We recommend choosing in exactly for leads that you agreed on calling at a specific time and after for the rest.
Note: By default, we ignore follow-up times for anything but call backs. If you want to set a follow-up time for a winner or loser (which can be useful if they are later migrated), you must select a follow-up that is different from the default. Archived leads are never set for follow-up.
Pro Tip: You can customise default call back times from your preferences. Click the user icon to the upper right and choose "My Preferences", then "Working".