The Idea Collection is the main blank area you see when you start a jam. It's the canvas on which you create your song. It consists of a collection of "Ideas", which are just stacks of loops that are played in sync. You can play an idea by clicking on it, after which you'll hear all the loops in the idea playback together.
To create an idea, first record something into the loop recorder buffer, and then click "Copy". If the idea collection is empty, the loop will be copied to a new idea, otherwise the loop will be copied to the current active (playing) idea.
Each time you copy a loop from the loop recorder, the loop recorder's buffer is cleared so you can record another loop freshly. However, you'll still hear the old loop playing because it will now be playing back from the active idea. This is useful because you can immediately start writing another loop that harmonizes with the old loop, allowing you to build complex melodies quickly without breaking your concentration.