Add to HDB, is used to easily register your intake of vitamin tablets and the like in the Health app (the iPhones Health Database).
Its easy to create a new tablet/product and to specify what vitamins (etc) that are included in it. Then, just click Add, to store the values you specified for the tablet, in the Health app.
Note that the app only stores the information (specified for a tablet) in the Health app. It does not use the stored information. That requires other apps. The stored values can be viewed using the Health app.