The default listener for Microsoft SQL Server. Clients connect here to run queries, and in a typical deployment that means the application servers and nothing else. Named instances additionally use UDP 1434 to tell clients which port to actually connect to.
A database should never be directly reachable from the internet. Exposed SQL Server instances are scanned for constantly and are a standard ransomware entry point. Bind to localhost or a private network and reach it through the application.
SQL Server reachable from the internet should be closed today. It is a primary ransomware entry point, and no amount of password strength compensates for direct exposure.
The port scanner will tell you whether 1433 is reachable on a host you control. Checking from outside your own network matters: a port can be open on the machine and still be blocked at the firewall, and it is the view from the internet that decides whether anyone else can reach it.
Only scan hosts you own or have permission to test.