What does "gag reel" mean?

What does "gag reel" mean?
A gag reel, also called a blooper or outtake reel, is a collection of mistakes and errors made during production of a film or theatre production.
These reels often contain flubbed lines, actors losing character and laughing, production problems like fires or props falling incorrectly, or practical jokes. Gag reels are often hilarious and an excellent way for audience members to step inside the production process and for production members to relive great moments. .....(see reference web for more information)