It's a commitment given by the buyer's bank to you, that they will make payment when you present a pre-agreed, specified set of documents to HSBC or a nominated bank in the UK.
You can reduce the risk of non-payment by the buyer's bank and associated country risks, by asking HSBC to add our confirmation to the document.
Funds can generally be advanced against compliant documents; or if documents aren’t fully compliant, after acceptance from the buyer’s bank has been received.