The Kaio-Ken was a technique developed by King Kai, and taught to Goku. (The name means, simply, "King Kai's Technique".) This technique enables the user to gain a boost to speed and power, at the cost of the user's vitality. The user becomes surrounded in a red glow when it is in effect. A single Kaio-Ken provides a slight increase- higher Kaio-Kens (referred to as "Kaio-Ken Times Two", etc.) multiply the user's power and speed by that number. The highest level of Kaio-Ken known is Times 20.
Possibly, it may be impossible to use the Kaio-Ken with the Super Saiyan level because the power would be too much for the user's body. If so, Goku was likely able to use his "Super Kaio-Ken" because he was already dead, and thus able to avoid such damaging side effects.