Medium Armor Focus is a star within the Constellation The Steed in Elder Scrolls Online.
This was added in Update 6 with the Champion System. It increases your Armor by [x] while wearing 5 or more pieces of Medium Armor. You can put 100 Champion Points into this passive ability for a maximum of 5280.
With each point invested, the increase in Armor diminishes. That is to say, for the first 10 CP invested, you would gain a 1003 increase in your Armor. However, the next 10 CP spent (making a total of 20 CP spent), will earn you only 897more Armor. This is 106 less than the increase gained in the first 10 CP.
As such, spending just 29 CP into Medium Armor Focus will already gain you a 2618 increase in Armor. This means you will have gained 50% of the Maximum increase possible (which is 5280) by spending just 29 out of the total of 100 CP. Therefore, beyond this point, the increase in Armor received per 1 CP used is very small and not as worthwhile.