This filter is designed to eliminate the pinkish/purple/reddish tint that certain black synthetic items might be subjected to when shooting under certain lighting conditions, such as incandescent bulbs. This filter can also serve to help protect the glass in your lens.
This is the construction of a type of filter. The type of process used to create a filter can affect its price. Filters can be constructed out of regular glass that sandwiches a coloured gel in between or in high-end filters, raw elements are added to the molten optical glass so there is no risk of uneven colour or fading.
When placed in the optical path, many filters block a certain amount of light from reaching the lens. Filter factor relates to how much exposure compensation is required in order to adjust for this. Most cameras have TTL metering and will be able to do this automatically.