SASE stands for Self Addressed Stamped Envelope. They are most often used in seed giveaways. How does it work? Read on:
You get two envelopes and two stamps. On the first envelope, you put one stamp, and then address it to yourself. On the second envelope, you put the other stamp, and put who you are getting the seeds from. Then you put the first envelope (the one addressed to you) inside the second envelope (the one addressed to the seed giver) and seal it. Then you mail the envelope.
When the seed giver receives the envelope, he or she opens it up, takes out your "self addressed stamped envelope," puts the seeds in, seals it, and drops it in the mail box.
A little note should also be included identifying yourself, the trade transaction, your forum name, and a short thanks so the other person can easily identify what the envelope is for and whom it is being sent back.