This article was posted some two years ago, but it's worth reposting.

Lightning invoices basics

Lightning Invoices are nowadays the standard for sending and receiving funds on the Lightning Network. If you want to receive some satoshis, you have to generate an invoice on your node/wallet. An invoice is nothing more than a text string (which starts with lnbc...) and contains, among other things, the recipient ID and the amount. There is actually more information but we won't go into that now. The invoice is most often shared using a QR code, which is nothing more than an image representation of the string described earlier.