SAE oil viscosity is a measure of how the oil flows under cold and hot conditions. Additionally, it is an indicator of how well the oil will cling to engine surfaces once they have reached their normal operating temperature. SAE, the Society of Automotive Engineers, created these classifications to inform the consumer about the operating conditions for which a specific oil can, or should be used. The SAE grading system allows users to select the proper oil for the conditions that it will be exposed to, and ensures that the engine it is used in will be protected.
For more info, see Understanding Motor Oil Ratings