In the House of Commons MPs must try to 'catch the Speaker's eye' in order to speak during a debate and MPs may speak only if called to do so by the Speaker. MPs try to attract the attention of the Speaker by standing, or half standing, to show that they want to speak. In the House of Lords the Lord Speaker does not call upon Lords to speak during debates as the order of speaking is pre-arranged.
Further information on this subject can be found from the following links.
About Parliament: Main business (debates)
Related glossary term: Lord Speaker
Related glossary term: The Speaker