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

The Farrier icon on the Home Screen will display badges if any horse has a farrier visit overdue (red) or imminent (amber). You can keep a record of farrier visits for each horse by tapping on the Farrier Icon on each horse's Home Screen.
You can also update a batch of horses via the same screen.
If you tap on the Home Screen icon when the badges are displayed, a summary page will show you which horse has overdue or imminent farrier visits due.
If you tap on the Home Screen icon when NO badges are displayed, you can select a horse record to branch to.
You can set the number of days warning you require on the 'Stables'->'Preferences' screen. If you don't want to be reminded, just set the days to zero. When you do that a circular red dot appears next to the top right hand corner of the icon, which reminds you that alerting is off.
Essential reading: Horse Home - Farrier Records.