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LISTEN registers the current Postgres backend as a listener on the notify condition notifyname.
Whenever the command NOTIFY notifyname is invoked, either by this backend or another one connected to the same database, all the backends currently listening on that notify condition are notified, and each will in turn notify its connected frontend application. See the discussion of NOTIFY for more information.
A backend can be deregistered for a given notify condition with the UNLISTEN command. Also, a backend's listen registrations are automatically cleared when the backend process exits.
The method a frontend application must use to detect notify events depends on which Postgres application programming interface it uses. With the basic libpq library, the application issues LISTEN as an ordinary SQL command, and then must periodically call the routine PQnotifies to find out whether any notify events have been received. Other interfaces such as libpgtcl provide higher-level methods for handling notify events; indeed, with libpgtcl the application programmer should not even issue LISTEN or UNLISTEN directly. See the documentation for the library you are using for more details.
The reference page for NOTIFY contains a more extensive discussion of the use of LISTEN and NOTIFY.
notifyname can be any string valid as a name; it need not correspond to the name of any actual table. If notifyname is enclosed in double-quotes, it need not even be a syntactically valid name, but can be any string up to 31 characters long.
In some previous releases of Postgres, notifyname had to be enclosed in double-quotes when it did not correspond to any existing table name, even if syntactically valid as a name. That is no longer required.