Rosin is any various resins applied to the hair of the bow to get the strings to vibrate. Rosin usually comes in a cake or a bar. There are special rosins out there for allergies, and various claims of special quality from their use. Rosin needs to be reapplied sparingly usually after prolonged use.
A good way to check if you need more rosin is to rub the bow across your arm. Be sure not to rub it across your fingers because the oil on your fingers causes the bow hair to go bad. If the test leave white dust behind, than rosin does not need to be applied. If there is little or no rosin left on your arm after you rub it, than it is safe to assume you need rosin on your bow.