Subgroups and Topics do similar things - they allow you to assign your members / parents to specific groups.
The main difference is that when users are assigned to Subgroups, they can't opt out of them. Once you're assigned to a Subgroup, only an Admin user can remove the user from it.
Topics on the other hand are groups to which users can decide to opt in or opt out. Topics are visible to users within the app by clicking on the 'Menu' icon at the top left of the screen (3 horizontal lines) and then by selecting 'Alert Preferences'. Here users can see the various topics and decide which they want to be part of and which they don't.
There is no difference between Subgroups and Topics when creating Alerts, News or Events. Both are visible and can be selected by the person sending the message.