The rule of 600 is the general rule of thumb when it comes to landscape astrophotography. It basically dictates the maxium exposure time based on the focal length of your camera lense. It helps you figure out the amount of time you can expose your sensor without creating star trails or oval stars as the earth rotates under the star field. It's worth noting that it is a starting point. Sometimes it's necessary to calculate the exposure time, then decrease it by a little bit.
The equation is simple: 600 / focual length = exposure time. For example, if your lense is 50mm, the maximum exposure you should take is 12 seconds (600/50 = 12). Here are some more examples: