There are so many things need to be down on greens when you play golf. It's such a subjective thing, learned from experience, that it's hard to describe it much further than that. Once you have hit your first putt and seen it break you begin to tune into things on a more subtle level, and the more time and experience you have putting on greens the more developed your ability to read them gets.
If you can see and balance you can probably read greens. All we're really talking about is being able to see variations from perfectly level. If you feel that you're hopelessly out of touch with this ability take comfort from the fact that at least you have enough of it to keep yourself from falling over when you're standing or walking. Just know that virtually everybody who can see and stand has the ability to read putts.
Reading a green or putt is imagining, visualizing, picturing, guessing, or foreseeing the line/path on which the ball will roll once struck. Some putts will be straight, but most of them will break -- some very slightly and some enormously.
Everyone wants to make the ball in the right direction to have it end up in the hole. Then you may need to get a sense of how the putt is going to curve or close. Reading the green will give you the visual information you need to get the ball started in the right direction to compensate for whatever break there is.
There are so many other factors influencing the break of putts plumb. Most players that plumb bob just do it out of habit. It's more of a pre-shot routine mannerism than anything that actually figures in the calculation of the break. And even if it helps suggest the general break it is vague on the order of saying that the sun rises in the east.
In addition, you may not know that judging a putt is good or not should combine speed and line. Gravity and friction act on the ball over time, so the more slowly the ball rolls the more it will break and vice versa.
As a golfer, and even a better golfer, you must read the greens to understand it more just like your golf clubs in hands. When you can enjoy your golf playing on the greens very freely, then you made it.