Bottom line
Your time in the Reserve is tracked in points, and points are what build toward a Reserve retirement. Each one-year stretch is an "anniversary year." Hit at least 50 points in that year and it counts as a "satisfactory" (good) year. Points come from the duty you do, and points from one year cannot be shifted to rescue another year.
What it means for you
If you are in the SELRES, you are expected to participate. If you are in the IRR, you generally do not have training requirements (musters are the exception), but you can still earn points other ways. Either way, the scoreboard is the same: points per anniversary year, 50 to make the year count. The rest of Chapter 4 builds on this, and the official record of your points lives in your Career Retirement Credit Report (CRCR).