You can do
lsof -n | grep -i "TCP\|UDP" | grep -v "ESTABLISHED\|CLOSE_WAIT"
to see all of your listening ports, but dollars to donuts that ntpd is running:
service ntpd status
And as for "What does socket in use" mean? If I can be forgiven for smoothing over some wrinkles (and for the very basic explanation, apologies of most of this is remedial for you)...TCP/IP (the language of the internet) specifies that each computer has an IP address, which uniquely identifies that computer on the internet. In addition, there are 65,000 numbered ports on each IP address that can be connected to.
When you want to connect to a web server, you open the site in your browser, but the machinery underneath is actually connecting you to port 80 on the web server's IP. The web server's daemon (the program listening for connections to port 80) uses a "socket" to hold open that port, reserving it for itself. Only one program can use the same port at a time.
Since you had ntpd running, it was using that port. 'ntpdate' tried to access that port, but since it was already held open, you got the 'socket already in use' error.
Changed to account for UDP as well