Bottom line
Inactive Duty (ID) is the duty you do when you are not on active duty. It has three parts: Inactive Duty Training (IDT, your drills and the extra drill types), Muster Duty (MD), and Funeral Honors Duty (FHD). This section sets the rules for every one of them: how long a period has to be, how many you can do in a day or a year, when you get paid, and how many retirement points each one is worth. Most of your year runs on these rules, so the numbers here matter.
What it means for you
A paid IDT period is at least four hours, and you can do no more than two in a calendar day. That is the floor and ceiling for normal drill. Beyond regular drills there are extra paid drill types (ATP, AFTP, RMP) with their own daily and yearly caps, plus several ways to earn points without pay (associate duty, appropriate duty, correspondence courses, MCJROTC help, referrals). If you are in the IRR, the big ones for you are musters (you can be ordered to one a year) and funeral honors. Read the caps before you bank on points: most of them are limited per day, per fiscal year, or per anniversary year, and the limits are not the same.