When looking at buying climbing ropes, they are rated to a number of "UIAA falls".
What does that mean?
Although I am not a lab technician it is important to understand the dynamics at play in these tests. Below is the best example I have read on the subject.
For the full article please read below.
NOTE: Also included is the section on "impact force", which is a factor of the fall height, weight of the climber, amount of rope "out" at time of fall, and rope stretch. It is important to know as it can directly effect your safety.
Fall Rating Half ropes have to pass a minimum of 5 controlled leader simulated falls. These falls are factor 1.77 falls so they are tested to close to the maximum fall that a climber can take (a factor 2). For those interested, a 55kg weight is dropped from a height of 2.30m above a preclipped karabiner. The amount of rope extending to the simulated belayer from the karabiner is only 30cm so that the set-up looks like the image on the left.
The weight is dropped with a 5 minute rest in between tests. The diagram on the side is a little out of date though as the test has now been modified so that the rope passes over a 0.75mm metal edge instead of through a karabiner. 3 sample ropes are tested and the weakest one of the three becomes the fall rating. The UIAA (Union Internationale des Associations d'Alpinisme) has a minimum fall rating of 5. It should be noted though that a factor fall of 1.77 is very high and chances are that you will never even have such a high factor fall all your life. So do not interpret this rating as the maximum amount of times you can fall on a rope before you have to chuck it. Also remember that taking huge whippers do not necessarily induce large fall factors (it is often the exact opposite) so don't just think that because you have taken 3 whippers on your rope that you should be starting to think about retiring it. For more information about fall factors you can click here Obviously, though, you will be looking for a rope that has a high fall rating as, in theory, it will last you longer. Theoretically thicker ropes will have a higher fall rating than thinner ropes, as you would expect.
The impact force measurement is the resulting force that is transferred onto the end of the rope- i.e. you. If you look at the diagram above again, you can see that the peak force generated by the 1.77 factor fall can be no more than 8Kn for a half rope. Basically the lower this figure is the better, not only because it reduces the force on you but also because it will reduce the force on the top runner- i.e. the lower the number the more force the rope absorbs. The flip side of this is that the rope will invariably have to stretch more to absorb more force. Now whilst this might not sound like a bad thing it can be. Situations can arise where you want the least amount of stretch in the rope- e.g. falling close to the ground, when top roping, etc. This is a point that is worth thinking about. If you are used to sport routes then you aren't going to be worried about minimizing the force on your top runner as it’s a bolt, and in addition the more the rope stretches the more you are going to have to climb back up if you fall. On the other hand if you climb a lot on badly protected terrain then the extra stretch may not be a worry if it helps keep the top runner from ripping out.
It means the rope is rated for X falls where the fall factor is 1.77 and the weight is 80kg. In layman's terms it's a pretty big fall with a pretty big guy, the sort of fall that if you took you most likely wouldn't want to climb again for the rest of the day! You may well never have such a big fall at all (in fact I'd hope you wouldn't!)
That said, the number is a rough guide and visual inspections should still take place regularly, especially on older rope that may have degraded over time. Equally because it's huge falls that the rope is rated for, don't feel that after x amount of (normal) falls you have to chuck it.
I'm sure I've seen some ropes with built in fall counters that work in terms of UIAA falls (measuring increments) if you feel that would be useful. In practice I wouldn't say it's necessary, but if you want one for added peace of mind then it might be something to consider.
Just to add to this answer, "fall factor" can be also thought of as this: