It generally is a reimplementation of https://signetfaucet.com in a very tiny way, while staying the same on the outside (and I hope even better since it does not use any captcha yet, I am thinking about visually limited people, and much more straightforward keyboard-driven UX).

Signet is a testing network of Bitcoin. Similar to Testnet, but not meant for testing the miners but rather developing on top of Bitcoin (with free coins first). Read more at https://en.bitcoin.it/wiki/Signet

TL;DR the blocks are not really being mined on Signet, but rather just signed by a chosen node (set of multiple if needed). For example Taproot was (as far as I remember) first activated on Signet, then Testnet and finally Mainnet (the real valuable Bitcoin). There is also a Regtest, but that is used for local testing and even though it can, it is usually not used for global operation. On the other hand Signet really follows the same rules as Bitcoin network, except that the blocks are not mined there. This brings some practical advantages over testing the development on Testnet where (and it happens sometimes) the blocks can take too long to get mined because someone was trying their super-new ASIC miner… (which is perfectly OK and that is what Testnet is meant for). HTH. Ask more if something is not clear.