A webhook is the server-to-server tap on the shoulder: when an event happens, payment seen, confirmed, completed, the platform POSTs the fact to your endpoint instead of waiting for you to ask. Polling is checking the mailbox; webhooks are the mail arriving.
The 'signed' part is what makes it infrastructure rather than a suggestion: each payload carries a cryptographic signature your server verifies before acting, because an endpoint that trusts anonymous POSTs is an endpoint anyone can lie to.
U.CASH fires signed webhooks on every payment state change against your confirmation policy, so inventory, access, and ledgers update themselves with verified events, no polling loops.
Every term behind the rails, the assets, and the settlement.