Allows you to have a point of contact for your server. This lets people using the server know how to contact the administrator. This information is available when the /admin command is used.
/as general set contact email@example.com
The contact field cannot contain any spaces. You should probably put a fairly self-explanatory email address in it. You should expect this email address to get all sorts of questions since many users may see it as a first place to turn if they cannot find out whom else to email. You might want to put contact info into a motd in addition, since it is more noticeable and you can use up more space.