Your question is one of the more sensible ones I've read up to date this week. Much better than the usual:
"I can't be bothered to use the search function so I'll ask this question here even though it gets asked 10 times a week. Which is more real, mirrors or cameras?"
"What The Best Cheapest PRofff Camera For Taking Pics At Wedngs Under 150 plz and tnxs!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!...
Your question is properly written and relevant.
The wider your aperture (the smaller the Fnumber) the faster you can have your shutter speed so "the faster the lens is".
So saying "It's an f2.8" is perfectly fine.
Let's say your lens is an F4 ... that means it's maximum aperture is F4 (you can shoot with a smaller aperture like f8 or f16 but the widest it will go if F4)
Now let's say you can properly expose the shot you are trying to do at the following settings.
f4, ISO 800, 1/15
1/15th of a second is REALLY slow, you'll probably end up with motion blur from motion in the scene or motion blur from your hands not being fully steady.
Now f2.8 is one whole stop more light than f4 (one full level of light) so you could speed up your shutter one full stop of light as well from 1/15th to 1/30th.
You gained one level of light from your aperture and lost one level of light by speeding up your shutter.
f2.8, ISO 800, 1/30th
With an f1.8 lens, you get yet another stop of light and could shoot at:
f1.8, ISO 800, 1/60th of a second.
Now, many consumer lenses are MUCH slower than that and as you zoom in, many of them will get "slower": their maximum aperture will shrink as you go futher in the focal lenght.
ISO works the same as aperture and shutter speed. Each doubling of the ISO gives you a full stop of light more so all of the following are equivalent in terms of exposure.
f4, ISO 100, 1/60th
f2.8, ISO 100, 1/120th
f4, ISO 200, 1/120th
f4, ISO 100, 1/30th
Each of those will give the same quantity of light ... there are going to be other differences in terms of depth of field, digital noise and motion blur but in terms of light, they are all equal.
Finally, if your lens has something like this:
70-300 f4 - 5.6
It means that when you are zoomed out (at it's widest angle) the lens can do f4 but as you zoom in, the maximum aperture will drop to f5.6.
I hope that made sense.