The PATH is simply a variable which is a colon-delimited list of directory paths to look for binaries to run, so you can type 'foo' at the command prompt rather than, say, '/opt/somebigpackage/bin/foo'.
Typically, you will have a basic PATH set for you at login, usually "/bin:/sbin:/usr/bin/:/usr/sbin". It is up to you as a user to add to this if you want. To add to it, you can follow the instructions you posted above:
What these two lines do is say: "make my path '/usr/local/Trolltech/Qt-4.1.3/bin' plus my current PATH, then export the results.
I would rather do it as:
Does the same thing, but in one line. Here, I put '/usr/local/Trolltech/Qt-4.1.3/bin' after the current PATH, as the PATH is searched from left to right until it finds a matching name, and presumably you will use the 'normal' unix commands more than the QT stuff. The 'export' is so that the changed PATH will be available to all new terms you open (otherwise the changes will only take effect in the current shell).
To make this change permanent (ie: for all you future logins) you must put it in a file that gets parsed at login such as ~/.profile or ~/.bashrc.