A Lightning invoice (BOLT11) is a signed payment request. On Bitcoin mainnet it starts with lnbc, followed by the amount and data such as a payment hash and description.
By default an invoice expires after 3,600 seconds (one hour) unless it sets another expiry. Each invoice is meant to be paid once.
For reusable payment codes there is BOLT12 "offers", starting with lno. Many wallets also support a Lightning address that looks like an email address; that is a convention run by a web server, not part of the BOLT specifications.