A flute is an instrument of the woodwind family which produces sound via the movement of air across an opening without the use of reeds.
Flutes are often used by Gleemen and Bards, the most notable being Thomdril Merrilin, to entertain their audience with a variety of songs, the tunes of which are often known by different names in different areas of even the same kingdom.
Thom Merrilin's FluteEdit
Thom's flute was a key part of Rand al'Thor and Matrim Cauthon's journey from Whitebridge to Caemlyn, enabling them to stay at inns or farm-houses and have warm meals to eat. Without Thom's flute they would have been forced to sleep in bushes and haystacks and hunt for food and they might not have made as good time as they did. If they had arrived in Caemlyn much later than they did, Rand would have missed seeing Logain and meeting Gawyn and Elayne in the garden of the Royal Palace; he wouldn't have met Morgase and Elaida wouldn't have had her Foretelling about him. With all these possible differences stemming from a flute, from Thom's choice to give Rand his instrument case, one can see how even small choices and the smallest of items can affect the weaving of the Pattern.
Later, Thom's flute was put to an even more important, and more dangerous, task: rescuing Moiraine from the Tower of Ghenjei. It was brought as an answer to the beginning statement of the common children's game Snakes and Foxes which cannot be won without cheating and is about the Aelfinn and Eelfinn. This saying goes "Courage to strengthen, fire to blind, music to dazzle, iron to bind."
There are several categories of flute; side-blown (transverse), end-blown, fipple, non-fipple, open and closed-ended. Each type of flute falls into three categories, for example, a Western Concert Flute is side-blown, non-fipple, and open ended (but closed at the blown end since it's side-blown).
It is not clear what kind of flute Thom Merrilin plays, but it is described several times as having gold and silver scrollwork indicating that it is made of metal. It is likely similar to a Western Concert Flute.