What does 30 means in 5w-30 engine oil?

The 5 and 30 refer to the relative viscosity of the oil when it is cold (5W) and when it is hot (30W). A 5W-30 oil behaves like a 5W thin viscosity oil when the engine is cold, and it behaves like a 30W thicker viscosity oil when the engine is hot. A very thin viscosity is undesirable in a hot engine. In very high temperature situations, an even higher viscosity range, up to 50W, might be called for. Always go by the engine manufacturers recommendations.