If your reply for the question above is “well my lens is pretty stable, it never run away so far”, then you kind of misunderstand the question. It is more related to the maximal aperture of the lens, or the smallest F-number (see this post for the calculation).
I used an example from my instructor in my photography class at MIT: In a sunny day, you want to have a sun tan by laying down in your room and open the windows for the sun to get to your skin. (The obvious question is why don’t you just grab your ass outdoor and enjoy the sun like normal people; but it was at MIT with a lot of strange people, so the example didn’t seem very awkward back then). Now, the average level of sun tan in your skin depends on how large you open the windows and how long you expose yourself, if the windows is wide open, then you just need to expose for a short time; inversely if you know that it is sunny only for a short time, you need to keep your windows wide open.
The same principle applies for photography. If you now that you’re your subject will be pretty fast, you need to expose your film for a very short time (aka higher the shutter speed) then you need to have a large aperture for a given ISO to have the same exposure. But you cannot increase the aperture forever, then there is a limit for your subject to be “fast”. This feature is very important for example when you take a photo at night at your highest ISO, then your choice of subject will depend largely on the f number of your lens. To sum up, the larger the aperture the lens can reach, the faster it is.
But why so complicated? To sound pro of course. if the question is “how fast the subject could be so that I can capture it nicely with this lens”, then it is pretty long and lame, or “what is the smallest f number of this lens”, which is too honest to enter the pro jargon.
Fun fact: the price of your lens increases exponentially with the small F-number, because it is quite tough to make a lens with large aperture.
So now facing the question: how fast is your lens? You can say proudly: well, it is a 1.0, pretty fast hein?; or in my case, “well it is a 4.5 you know, shitty lens you know”.