An urgent debate is called at short notice in the House of Commons on a subject of a "specific and important matter that should have urgent consideration". An MP may apply to the Speaker for an emergency debate under the rules of Standing Order No. 24. Many more MPs request emergency debates than are granted. The Speaker usually grants no more than one or two a year and limits them to matters of national importance. They take place within 24 hours of being granted.
Further information on this subject can be found from the following link.
Parliamentary Information List: Urgent debates since 1979