Path is a set of directories stored in a variable. You can watch yours by issuing
echo $PATH
in the CLI.
If you launch a program e.g. GIMP or anything else, the system will look for that program in those dirs, and if it's not there then you will meet the famous "No such file or directory" message.
About Sunbird, if you still want to install it I think you should install some mozilla package as well, I think it depends on them. See the error message I have posted before.