TL;DR: it's basically phoenixd but you don't even have to run phoenixd on a server alongside your actual app (say, a store or a blog). Instead, your LN node runs on-demand inside your Nextjs app, on the backend.

Your node is connected to one of MDK's always-on nodes, which provides inbound liquidity and "wakes up" your node when a payment is incoming.

Received payments stack up on your side of the channel. You can then decide to transfer them to another node (via LN Address or Bolt12) from your MDK account, on MDK's website. The MDK infrastructure then issues a request to your MDK node, which only pays if the LN Address/Bolt12 matches one that you provided during the initial config. This ensures this setup is actually self-custodial, since payments can only be sent to whitelisted destinations. You can also, of course, recover on-chain with the seed phrase and some (yet unreleased?) recovery tool that should basically tell the MDK remote node to force close the channel.

I think a cool thing to add for merchants would be on-chain payouts (ie. withdrawing from your "serverless" node to an on-chain address). For example with splicing and by specifying a xpub during the initial configuration.