socket already in use
An example is a process listening on port 80. It will make it impossible to start a typical web server because port 80 is the default web server (http or www) port. In order to start a web server, you will have to kill that other process. A socket is the combination of an IP address and a port number.
To kill a process blocking a socket first find the socket you are interested:
"a" for "all," "v" for "verbose," and then "p" for "process" (show process #). For example, you could find a line
tcp 0 0 *:www *:* LISTEN 6670/apache2
This means that the process with the id 6670 listens on the www-port.
tells you that the www-port is port 80:
www 80/tcp http # WorldWideWeb HTTP
Then kill the respective process:
kill -9 6670