One micron (or micrometer) is one millionth of on meter or approximately 1/25000 of an inch. It is often represented by the symbol "µm ".
To give you an idea of just how small a micron is, a speck of dust is usually 500 microns in size. A grain of ragweed pollen is about 20 microns wide. Many forms of bacteria can be less than 1 micron long.
Water filters are rated in microns regarding the smallest possible particle they can remove. Just about every refrigerator water filter has an average micron rating of 20 microns, depending on what type of filter media it uses.
When looking for the best possible water filter, the rule of thumb is that the lower the micron rating, the better.