Points of issue are the small, specific details that tell one printing, issue or state of a book from another: a misprint, a missing line, a particular binding colour, a price on the jacket or a list of titles on the back. Bibliographers and dealers record them so that copies can be identified with confidence.
How to spot it
Points are particular to each book, so you need a list for the title in hand. Each point names a place, for example "page 53, line 17", and what to look for there, such as a repeated word or a misspelling that was corrected in later copies. Jacket points might be the price on the flap or the reviews on the back. Check every point listed, because a book can have been made up from mixed sheets.
What it does to value
For famous books, points can separate a copy worth a great deal from one worth very little, because the first issue may be a small fraction of the first printing. A copy that matches every point of the first issue commands the premium; one that fails a key point is priced as a later issue or state.
MyVolumes lists points only where two independent sources agree. Read points of issue explained and use "Check my copy" on any researched edition page.