PBase

I'm a beginner, so this question might look stupid.
What does "f/2.8" or "f/4.5-5.6 II" mean?

This is the 'result' of a ratio that indicates the aperture (lens opening). The closer the ratio (f/2.8 vs f/4, for example) the wider the opening. The wider the opening, the less depth of field - that is, a shallower 'zone' of the subject matter will be in focus.

Full stops measuer twice as much (or half as much light): 2, 2.8, 4, 5.6, 8, 11, 16, and 22 are full stops. Many of the best lenses can go one full stop wider at f/1.4, which pros like because of extremely shallow depth of field, like you might see in some food photography.

Wider apertures are indicative of higher-quality glass, and give the photographer a brighter viewfinder.

Aperture is always balanced against shutter speed. Thus f/2 at 1/500 sec gives you the same exposure as f/4 at 1/250 sec. In the latter example, the aperture is 1/2 of the first setting, but the shutter speed is is cut in half to compensate for the 'loss' of light coming through the now smaller aperture.