Dawlet is a toy DAW (digital audio workstation) whose interface is a webapp. It's just a proof-of-concept.
You wouldn't. You'd use Ardour or Reaper or Bitwig or something. You could, in theory, record a song in it, but it lacks scrolling, waveform display, punching, editing, plugin support, sends, exporting, MIDI, and no doubt many other things.
There are two threads in the program. One is the "audio thread", which is the realtime thread to play, mix, and record audio. The other thread (called the "DAW thread" for lack of a better name) does everything else -- it's the webserver, and it reads and writes from disk. The two threads communicate solely by sending messages back and forth (through ring buffers). The audio thread is not allowed to do anything that can take unbounded time, and that includes allocating memory, so the DAW thread needs to keep it supplied with chunks of memory, usually in the form of audio buffers. During playback, it has to read-ahead enough audio from the disk so the audio thread will have it by the time it needs to play it.
As a webserver, the DAW thread serves up the HTML/JS/CSS, handles a REST-like API for making changes to the project (such as adding a new track), and handles transport commands ("play", "pause", "record", etc.) from a Websocket.
Both threads need to access the project data structure, mainly because only the DAW thread can save it to disk. The audio thread has the responsibility of making changes to the project. Let's consider what happens when a new track is added to the project. The DAW thread allocates the Track object, then sends it to the audio thread (along with information about where to put it). The audio thread can make the change quickly when it gets the message (so it doesn't jeopardize keeping the audio going), but until that happens, it's not safe for the DAW thread to access the project structure. So the DAW thread keeps track of whether it has outstanding "mutation" requests to the audio thread, and will wait until they've all been performed and acknowledged before it tries to walk the project structure to write it to disk.