I saw that every stone has its own levels or i dont know its right name..
I'v seen it has 1000, 2000, etc...
what is the meaning and how important it is?
Grit levels. The higher the number, the smoother the stone. However the grit levels are not all the same and vary from mnfctr to mnfctr, but are kinda/sorta in the same ballpark range.
Generally you'd use a very coarse stone like 200-400 to re-shape a broken tip or to put a fresh bevel on, 800-1000 is for re-establishing the bevel, and anything from 2000 and up for polishing.
Caveat: The whole sharpening and polishing process is highly personal, there are very few fast and hard rules. There are many types of sharpening mediums: Waterstones, oilstones, diamond stones, ceramic stones, sandpaper, diamond paste, etc. Each one of these has it's good points and bad points. Again, choice of medium is highly personal.
There are several different kinds of "stones". Some of the ones you commonly see are Arkansas and Coticules, and Norton Cryolon. Arkansas are grouped into rough categories; they can give you a good polish but work pretty slowly. Another kind you see are diamond abrasives such as DMT Dia-Sharps. They're categorized by the size (in microns) of the diamonds in the matrix. Sometimes the older stones are referred to as oil stones or whetstones. Water is generally better than oil, even with those types of stones.
Water stones are the best tools for sharpening good kitchen knives. There are natural ones and synthetic ones. Synthetics are rated by the size of the particles (Aluminum Oxide, or Silicon Carbide). The binder can be resin, clay or magnesia. They are roughly broken down into arato (coarse), nagato (medium) and shiageto (fine/polishing). I apologize if my spelling is a little off; just got done getting run over on the line tonite...really really busy shift! The arato will generally be under 1k, and the polishers begin somewhere around 4/5k depending on who you ask. The number can mean micron or mesh, depending on the source of the stone (ie American or Japanese). Natural ones are grouped into the same categories but there's really no way to assign a number to them since the grits vary in natural stones. J-nats are named by the mine where they're dug, the appearance of the stone and the layer from which they were mined. Thus a Hideryami Renge Suita would be a red-speckled stone from a given layer of a specific mine. J.Broida can tell you way more about naturals than I can, but that's really a much more involved topic than we can cover here.
I'm kind of oversimplifying a bit but this is the Cliff Notes version. You could fill several large volumes with sharpening info!
Many/most folks will start off with a 1,000 grit stone, also called a '1k". If a knife is badly chipped sometimes a coarser stone will be employed. Lots of people don't ever go above 4,000 or 5,000 grit for kitchen knives. Personally I like to finish good knives at 8,000-10,000...sometimes higher. I will say that IMOHO water stones are the only things you should use for kitchen knives. Arkansas are kind of obsolete (no offense to those who like them). The bulk of sharpening could be done with just a 1k and a 4k, 5k or 6k. I like to use more though...way, way more!
Powered gear (eg belt sanders) is also very effective, provided you know what you're doing. It's a very fast way to sharpen, and a very convenient way to ruin a knife much faster than you could on a stone. Belts are usually paper, plastic film or fabric coated with aluminum oxide, silicone carbide or ceramic. Diamond (poly- or mono-crystaline) and CBN (cubic boron nitrate) compounds are used over linen or leather belts for sharpening and polishing. A leather belt is often used to deburr and hone. There are other kinds of powered tools that can be used such as paper wheels and buffers but they have limited utility for kitchen knives.
Philosophically I don't really like to use a grinder for quality Japanese knives. But it will work. Often I sharpen entry level quasi-Japanese knives like Shun and Global on belts. A skilled user can get a hair popping edge on a grinder very quickly.