Pairing is an arrangement where an MP of one party agrees with an MP of an opposing party not to vote in a particular division. This gives both MPs the opportunity not to attend. Pairing is an informal arrangement and is not recognised by the House of Commons' rules. Such arrangements have to be registered with the whips who check that the agreement is stuck to. Pairing is not allowed in divisions of great political importance but pairings can last for months or years.
Further information on this subject can be found from the following link.
The Whips Office (Commons Library note)