Port 1433: Microsoft SQL Server
Port 1433 is the default port for Microsoft SQL Server databases.
What runs on port 1433
SQL Server listens on TCP 1433 for database connections, and the SQL Server Browser service uses UDP 1434 to tell clients where named instances are. It is common in businesses running accounting, point-of-sale and line-of-business software.
On a home network
Home devices do not use it. Small offices sometimes run SQL Server Express on a PC for accounting or practice-management software.
If port 1433 is reachable from the internet
Databases should not be reachable from the internet. SQL Server on 1433 is brute forced heavily, especially the "sa" administrator account, and compromised servers are used for cryptomining and ransomware. In 2003 the SQL Slammer worm infected tens of thousands of SQL servers within minutes through the neighbouring UDP port 1434.
How to check whether it is open on your network
Check whether any port forward points at 1433 on an office PC or server. RECON's Exposure Check shows whether it answers from outside; business software can usually be set up to work over a VPN instead.
To find out whether port 1433 is open on something in your home, and whether the internet can reach it, the app scans your Wi-Fi from your phone and checks your public address from the outside. It says what it checked and what it could not. Get RECON for iPhone — one-time purchase, no account, nothing leaves the device.