The reed seat refers to where any reed is inserted/sits in the bagpipe. The chanter has a chanter reed seat, and each drone has its own drone reed seat. The reeds should always be hemped such that the reed seat is a nice, snug connection.
Andrew is a prolific practitioner of the bagpipe, having been active at the highest level of pipe bands, solo competition, teaching, and creative endeavors for the past 20 years. He's also the founder and creator of Dojo U and of PipersDojo.com