Your Minimum Point is the number of an item you ideally want to have in stock or on the way at any given point.
For example, if you always want 10 copies of Watchmen in your store or on order, you'd enter 10 in your Minimum Point field. If you sell down to 3 copies, and Comic Shop Assistant knows you have two on order from Lunar, the next time you gather an order by minimum points for PRH, CSA will let you know that you should order 5 copies in order to reach your Minimum Point.
Keep in mind that some back orders will expire after a specified amount of time, and will not be included in your Minimum Point calculations. You can control a distributor's back order cancellation timeline within that distributor's contact record, under the ORDERS tab.