I use 3rd party library in my asp.net application. Then application will be deployed on client server. Owner says: a Developer License licenses only ONE developer to create ONE .NET application using their library and Developer License is not royalty free. What does it mean that it's not royalty free?
It means you cannot distribute the libraries without paying the vendor a license fee for each deployment/ client distribution.
Sometimes this is because the vendor is an OEM partner with another company that licenses software/IP/libraries to them, where they in turn pay a royalty. I found this to be the case with some PDF libraries.
It means that you need to pay for every installation/deployment. A royalty free license means that you pay once for the license and then you are allowed to install the software as many times as you wish.
"Royalty free" means that you don't need to do additional payments to the library maker when you distribute an application that make use of the library. If it is not, it means that you need to pay a fee to the library maker for each copy of your application that you distribute.
None of the other answers so far is exactly correct. From a practice standpoint, it can mean all or none of those things, depending on what the producer of the library wants as part of a license.
By saying it's "not royalty free", he's emphasizing in no uncertain terms that it's a rights-managed license, which is essentially the converse of a royalty free license.
Generally, the fees due through a rights managed license scale with the utility gained from the use of it. This contrasts with a royalty free license, which generally grants rights in perpetuity without need for further compensation.
Whether or not you have to pay per deployment will depend on the specific rights that the licensor grants you. All that statement means is that there may be such additional charges. A scaling per-deployment royalty is one of the more common fee schedules used with software rights-managed licenses, but it is by no means the only one.