N.B. The screenshots and descriptions are for the iPhone/iPad version.

This is where you can set the number of days warning the App gives you when certain events are imminent or indeed are overdue.
The App warns you via badge alerts on Home and secondary screen icons and also on Horse listings. The badges are amber for imminent and red for overdue.
For horse related warnings such as Vaccines, Worming and regular professional visits, you should set up the interval these events need to happen in the records for each horse.
e.g. If you need to give 'Dobbin' a flu jab every six months and want to be reminded 14 days before it is due, set 14 days in the Preferences screen (this will apply to all horses) and select an interval of 6 months under the Vaccines options on Dobbin's Home screen. You will be reminded 14 days before the day 6 months after the last jab.
If you do not want to be reminded of for example 'Physio' appointments for ANY horse, set the Physio days to zero in the Preferences screen
If you want to be reminded of for example 'Physio' appointments for some horses, set the Physio days to (say) 14 in the Preferences screen and the Interval to zero for the horses you do NOT want to be reminded about.
You can set here, which units of weight and height you use in your Stables.