A writ is a document which orders that an election be held. For an election of a member of parlament in England, it is worded like this:
Charles the Thirdby the Grace of God of the United Kingdom of Great Britain and Northern Ireland and of Our other Realms and Territories King Head of the Commonwealth Defender of the Faith To the Returning Officer for the [X] Constituency Greeting Whereas Parliament has dissolved We Command you that due notice being first given you do cause election to be made according to law of a Member to serve in Parliament for the said [X] Constituency And that you do cause the name of such Member when so elected whether he be present or absent to be certified to Us in Our Chancery without delay Witness Ourself at Westminster the thirtieth day of May in the second year of Our Reign in the year of Our Lord Two thousand and twenty-six
The writ contains some further information, such as the clerk of the crown in chancery (the head of the Ministry of Justice) and miscellaneous details under the text.