The term "server busy" means that currently there are many members logged in, using the features of the site and doing their different stuff. When there are too many members logged in the site, there is a possibility that the site servers (the things that hold on the site on line) will get busy. Basically, when the severs get busy everything gets slower, and sometimes even new logs from other members are begin blocked for few hours. There are also many other effects of busy servers, but i don't think that they are important for now.
Just try to log in after few hours, maybe at such hours that you know that many people are at work / busy / sleeping, etc.
As you know as humans we have many other things to do, so just take your time and do other stuff if you are even a bit busy. I am sure that everyone has unique and busy lives outside.
This thing never happens too much, but patience and waiting is the key here.