What does it mean to flute?

Answer 1

Well Flute is a musical instrument but as you are asking about an action then obviously you are talking about making a pie which has got a method Fluting.
Fluting there in making pie or any kind of modeling is that you make wavy shape (pattern) all around your model or pie.
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Answer 2

To "flute" is a way of shaping a pie crust in a decorative design, which is actually quite easy to do. After placing the raw pie crust in the pie pan, you simply place your first two fingers on the inside edge of the pie crust, with your thumb between them on the outside edge of the pie crust, then gently press the raw crust with your thumb between your two fingers. Do this all the way around the crust until the entire crust has been "fluted".

If you are baking a two crust pie, such as a fruit pie, you do it the same way, just do it after you place the top crust on. This will also help seal the two crusts together and prevent the juices from leaking out while baking.