title_body: What exactly is a Redeemer?
upvoted_answer: From https://iohk.io/en/blog/posts/2021/04/13/plutus-what-you-need-to-know/:
  
  When validating an address, the script will access the data being
  carried by the output, the transaction being validated, and some
  additional pieces of data called redeemers, which the transaction
  provides for every input. By looking up all this information, the
  script has enough context to give a ‘yes’ or ‘no’ answer in what can
  be highly complex situations and use cases.
  
  In lecture 1 of the current plutus pioneers program, you find a redeemer on the auction validation script. When making a bid, there are essentially five cases to check. All these five cases need to pass or the transaction will not be even committed to the blockchain (which is good, because you're saving on fees for something that won't succeed anyway):
  
  Is the bid sufficient (the bid is &gt;= the minimum valid bid)
  The auction hasn't closed
  
  The three other cases also do data checks (output datum/value, bid refund).
  The redeemer (in the auction example) itself is a simple data type that we can define (AuctionAction) to do data validation (particularly on inputs).