A laissez passer is a permit or another form of official authorization to go somewhere. You'll need a laissez passer to go backstage after the concert and meet the band. Rock on!
In French, laissez passer means "let pass," and the phrase is used for documents that give a person permission to pass. Usually, a laissez passer is required for passage between countries for specific reasons. International organizations like the Red Cross issue a laissez passer to people assisting with a humanitarian crisis, for example; it works as an emergency passport.