What’s dual licensing i’m kinda interested
A license in the software sense is effectively the legal terms and conditions for using the software in any way. A license can define anything your legal mind can dream up: "only usable when you wear a red shirt," "only usable on the third Tuesday of the month," etc. You can multi-license things: "License A is that you can only use this software when you wear a red shirt. License B is that you can only use it on the third Tuesday of the month. This software is dual licensed under A and B: you can choose which license you want to use, but you must use one of them if you want to use the software legally, because those are the legal conditions I've laid out in using the software." If you use the software on Wednesday wearing a blue shirt, you're operating it against the terms and effectively are in breach of the license. This is (part of) why putting source code out on the internet isn't the same as "open source" and why downloading and using source code you find without reviewing the actual license terms may end you up in hot water. It's why standardized licenses are so helpful to legal teams that review these sorts of things at corporations.
These are obviously facetious examples, but most for-profit entities might choose a dual licensing structure where "if you want to run it for free, you choose something in the realm of open source licenses and if you don't like the terms of those licenses, you pay us for a license that gives you something else." In cases like the blog author's here: you say "option A is AGPL" and "option B is you pay us for a license to remove AGPL and which gives you different rights." You hope enough companies are scared by AGPL to want to pay you for the non-AGPL version and the distribution/OSS-friendly nature of AGPL helps you build a user-base enough that by the time it gets to a legal team to approve/deny, that the legal team denies and is willing to pay to make the problem go away.