The classic mixing interface is a row of identical channels with numbers on top, and that presentation hides exactly what needs understanding: that the elements of a song occupy positions, and that two elements in the same position compete. Masking is hard to see in a column of decibels.
Mix Space turns each stem into a sphere on a stage. Drag it sideways to pan it; pull it closer or push it away to bring it forward or send it back. These are the same decisions you'd make with a fader, a pan pot and a reverb send, but placed where the brain understands them without translating. And with the stage in front of you, stacking three elements dead centre stops looking normal.
The material comes from the Artist Pool: songs by real independent artists, with their stems separated. Practising on actual music matters more than it sounds, because synthetic examples are too clean — they lack the overlap, the room noise and the arrangement decisions that make mixing hard. You can also upload your own stems.
The reasoning behind moving a sphere sideways is in the stereo image guide, and what happens when you push it back, in reverb and delay. You can meet the artists you're hearing in the artists section.