A lot of Sigmas lenses have the word macro slapped on them primarily as marketing. They do not truly reach macro capability, some don't even come close. You could use extension tubes to make your lens work for macro work, depending on how far off from macro it is. You could also use a diopter set on the front of the lens, however extension tubes will cause no drop in image quality while the same cannot be said for diopters.

Otherwise, Sigma & others make macro lenses you can use. I believe Tamron is supposed to have a better macro lens than sigma, it's their 90mm 2.8 macro if I am remembering correctly and it's in the ~$300 range perhaps.

there isn't any zoom on that lense. a lense that has a hyphen between two numbers will be a zoom lens. it would be like 75-300mm.

90mm is the focal length of the lens, it is a fixed focal length lens. Just like you have an 18-200 mm lens, yours is a zoom however.

The 2.8 is the aperture, which describes the size of the opening in the lens. It is essentially computed by dividing the focal length by the internal diameter. It tells you how efficiently that lens can gather light, the smaller the number, the larger the physical opening or aperture in the lens is, and the more light it can gather.

Well larger apertures (smaller f numbers) cost more money, because the glass is larger and it complicates the design to maintain good performance. People pay a hefty premium to have "fast" large aperture lenses because they work better than other lenses when stopped down (stopping down a lens basically always helps and the larger your maximum aperture is, the larger your aperture will still be when stopped down two or three stops. An f2.8 lens stopped down to 4 or 5.6 will likely outperform another lens of the same focal length whose max aperture is 4 or 5.6). Larger apertures let you shoot in lower light without boosting ISO and let you play with the depth of field more.

For some people they don't need the extra stop or more that more expensive lenses provide so spending the extra money isn't worth it, and for others it can be essential. A larger aperture certainly leads to a heavier lens.