Bad Snacks, producer and host of Ableton's official beginner series, gives a quick orientation to the two core views in Ableton Live. Session View is the default on launch, built for working with loops and for performing electronic music live. Arrangement View is where linear track-building happens, and toggling between the two is as simple as hitting the Tab key.
In Arrangement View, MIDI tracks and audio tracks sit side by side. MIDI is a digital language that tells virtual instruments what to play, but it produces no sound until an instrument is assigned to it, which is the starting point for building drums in this series.