Port 80 (as Rigga said) is used for Web Servers (eg IIS, Apache).
However, there may be other programs usin Port 80 on your computer for a web based interface. If you have any programs with a remote web based interface then you may need this port.
On your computer try going to localhost:80 in Internet Explorer. Then you can see whats actually running on this port if it is bieng used for a web interface. If there is nothing running then it is probabley open for no reason and you can close it.
On the other hand, allthough the port 80 is used for HTTP i *think* if you really wanted to, another service may be set up to use port 80 although it isnt a webserver. Perhaps a virus is using this port as most people would not think to try and block it seen as its use is for HTTP generally.
If you dont know why its open in the first place then your most likley able to close it.