The prelude is background music played from the time wedding guests begin arriving to the wedding ceremony until the wedding party is ready to begin the procession.
Merriam-Webster Online Dictionaryprelude (noun)
an introductory performance, action, or event preceding and preparing for the principal or a more important matter
a) a musical section or movement introducing the theme or chief subject (as of a fugue or suite) or serving as an introduction to an opera or oratorio
b) an opening voluntary
c) a separate concert piece usually for piano or orchestra and based entirely on a short motif
to serve as a prelude to
to play as a prelude to give or serve as a prelude , especially to play a musical introduction