The boiling point is the range of temperatures at which the components of a liquid hydrocarbon is converted into a vapor.  For a pure stream, this would be at a single temperature.  However, as most hydrocarbon streams in a refinery are a mix of different components, they tend to boil across a temperature range.  At the low end is the initial boiling point where components first begin to vaporize, and at the high end is the final boiling point where the last components to boil vaporize.