What are payment IDs and how do I use them?
A Payment ID is a label that you can attach to a transaction before sending it so that the recipient can cross-reference it with its own incoming transaction history and attribute a specific amount of TUBE coins received to your unique transaction.
The recipient of your transaction might need you to include a specific Payment ID in order to verify that a given transaction is coming from you specifically: since with Cryptonote nobody can see the origin and destination addresses of a transaction (also the amount sent is obfuscated), if someone receives many payments at the same destination address, then the only way to tell which payment comes from who, along with the relative amounts, is to rely on Payment IDs.
Then again in the GUI Wallet, in the History tab, you can see the transaction hash and the payment ID (if given) for each transaction: you can look up your transaction hash or payment ID on the public explorer to get information about the status of your transaction.
Important: most exchangers require a specific payment ID in the transactions from and to the exchanger. If the payment ID is incorrect or not specified you may lose your coins from that transaction.