Normally, a project has but a single sequence, as shown by the tab above the timeline and under the Sequences tab in the bin (top-left).
Sequences can be embedded in other sequences, as noted above, where they become just another track.
Often, a sequence has many tracks. Higher tracks take precedence over lower, so text clips, on a higher track, will be displayed over content on lower tracks. And text clips, just like any other, can be moved by dragging.
A track can be locked (so it isn't affected by work on other tracks) by right-clicking at the left of the track and making that choice.
