This license expires in one year after purchasing IDM. It means that IDM stops working and asks to enter a new product key in one year unless the license is paid and extended for another year. In general this license is one-time payment.
If you see a warning message about using IDM on other computers when you try to install it on a new computer, you need to uninstall it from all those computers if it is possible. If you are sure that IDM is not installed or will not be used on any of those computers again you can just ignore this message and continue registration. Otherwise, IDM will detect that it is being used on several computers and your serial number will be blocked.
If you want to move IDM on another computer, you need to uninstall IDM from the old computer first if it is possible. If the old computer is broken, stolen or formatted it is also OK. Then you may install and register IDM on a new computer.
Please note that IDM has per computer license. In order to use IDM on several computers at the same time please obtain licenses for each one.
If you often reinstall your system on the same computer, please download and install the latest version of IDM. It has additional binding to the hardware and will not block your serial number if you use it on one computer. Also, it is always better to use the same computer name after each reinstall.