An SDK is a "software development kit", or a set of code blocks that you can use to build different software, by combining them differently or changing their parameters.
Discussion
We have plans for what to build with them, but we made the choice to start off extremely modular/reusable because we want to build an entire suite of clients and be able to maintain and alter them later, as a group.
Additionally, in the spirit of contributing to the Nostr community at large, this lets us make our work available for other developers to use from the get-go. As we use it, and possibly as other devs use it, we'll incorporate feedback and fix bugs to make this foundation every more robust and stable.