A subsidiary title is an hereditary title held by a royal or a noble but which is not regularly used to identify that person.
For example, the Duke of Norfolk is also the Earl of Arundel, the Earl of Surrey, the Earl of Norfolk, the Baron Beaumont, the Baron Maltravers, the Baron FitzAlan, the Baron Clun, the Baron Oswaldestre, and the Baron Howard of Glossop. In day-to-day practice, the individual who holds all of these titles would be referred to only by his most senior title - in this case, "Duke of Norfolk" - while all of his other titles would be subsidiary titles.
In the United Kingdom, a noble's heir apparent may use his parent's most senior subsidiary title as a courtesy title, provided that it causes no confusion. For example, the Duke of Norfolk's heir apparent is known as "Earl of Arundel", although the son does not technically become Earl of Arundel until his father's death and is legally still a commoner.
If a subsidiary peerage has the same name as a higher peerage, it is not used as a courtesy title. For example, the Duke of Manchester is also Earl of Manchester, but his heir is styled "Viscount Mandeville".
An heir apparent can be summoned to the House of Lords, before his parent's death, by a writ of acceleration – that is, by accelerating the inheritance of a junior title (usually a barony). For example, a writ of acceleration could cause a courtesy Earl of Arundel to inherit the Maltravers barony prematurely, whereupon he would join the House of Lords as Lord Maltravers.