You are purchasing one or more licenses to use the software. This is granted in the form of an unlock key per license purchased that turns your trial version into the full version, removing any limitations. These keys are sent by email and are valid for an unlimited amount of time for the particular version purchased.
You will also be entitled to free minor version updates to the software. So if you purchase version 4.0, you will be entitled to all 4.x updates. Major version updates (for example to 5.0) may require an upgrade charge.