The sequencer workspace is horizontally striped into channels and each video strip will go in a horizontal channel. Each channel is numbered consecutively on the Y axis, starting from zero. The X axis represents time.
The first channel 0 is unusable as a place to put strips. This is because it is used by the Sequencer Display to show a composite of all strips above channel 0.
The channels are stacked bottom to top (the lowest channel forms the background and the highest the foreground).
By default the Sequencer is enabled however, it can be disabled in the Post Processing Panel.