Discord and matric are room based chat services. You join a room and there’s a chat around some subject going on which you can participate in. I use it for my DND group to discuss upcoming games and our general lives. Some larger communities form on these apps. Say, game emulation, people who talk about a video game, college members and alumni, people in a city, weed in a country, youtubers, etc.

This seems to be a bridge that copies messages from discord and sends them to matrix, or copies messages from matrix and sends them to discord. Allowing users from one service to talk to the other. The intent is to slowly bring members of a larger discord community over to the matrix server.

Gpl licences don’t mean free as in no payments allowed, but free as in freedom. Many open source, even gpl, software is paid. All agpl, or gpl in general, means is that the code is free to copy as long as one abides by a set of pro-consumer rules while making your changes open too. It gives specific rights to the user or person who copies the project

This means you could build or host this product for free if you wanted, taking the code and setting it up yourself, but their distribution of the code or hosting of their project can include a paid tier or payment upfront. As long as they give all code for the project to people who use it they’re not in violation of the agpl