The Web server (running the Web site) is currently unable to handle the HTTP request due to a temporary overloading or maintenance of the server. The implication is that this is a temporary condition which will be alleviated after some delay. Some servers in this state may also simply refuse the socket connection, in which case a different error may be generated because the socket creation timed out.
The Web server is effectively 'closed for repair'. It is still functioning minimally because it can at least respond with a 503 status code, but full service is impossible i.e. the Web site is simply unavailable. There are a myriad possible reasons for this, but generally it is because of some human intervention by the operators of the Web server machine. You can usually expect that someone is working on the problem, and normal service will resume as soon as possible.
Please contact the system operators of the Web site (e.g. your ISP) to determine why the service is down. They will be in a much better position to help you than we are for this type of error.
Any client (e.g. your Web browser or our CheckUpDown robot) goes through the following cycle when it communicates with the Web server:
This error occurs in the final step above when the client receives an HTTP status code that it recognises as '503'.