A curtain raiser is a performance

Performance

A performance, in performing arts, generally comprises an event in which a performer or group of performers behave in a particular way for another group of people, the audience. Choral music and ballet are examples. Usually the performers participate in rehearsals beforehand. Afterwards audience...

, stage act, show, actor

Actor

An actor is a person who acts in a dramatic production and who works in film, television, theatre, or radio in that capacity...

 or performer that opens a show for the main attraction. The term is derived from the act of raising the stage curtain

Curtain

A curtain is a piece of cloth intended to block or obscure light, or drafts, or water in the case of a shower curtain. Curtains hung over a doorway are known as portières...

. The first person on stage has "raised the curtain".

The fashion in the late Victorian era

Victorian era

The Victorian era of British history was the period of Queen Victoria's reign from 20 June 1837 until her death on 22 January 1901. It was a long period of peace, prosperity, refined sensibilities and national self-confidence...

 and Edwardian era was to present long evenings in the theatre, and so full-length pieces were often presented together with companion pieces. Each full-length work was normally accompanied by one or two short companion pieces. If the piece began the performance, it was called a curtain raiser. One that followed the full-length piece was called an afterpiece. W. J. MacQueen-Pope

W. J. MacQueen-Pope

Walter James MacQueen-Pope was an English theatre historian and publicist. From a theatrical family which could be traced back to contemporaries of Shakespeare, he was in management for the first part of his career, but switched to publicity, in which field he became well-known...

commented, concerning the curtain raisers:

This was a one-act play, seen only by the early comers. It would play to empty boxes, half-empty upper circle, to a gradually filling stalls and dress circle, but to an attentive, grateful and appreciative pit and gallery. Often these plays were little gems. They deserved much better treatment than they got, but those who saw them delighted in them ... they served to give young actors and actresses a chance to win their spurs ... the stalls and the boxes lost much by missing the curtain-raiser, but to them dinner was more important.