The tooltip describes the relative constraint between 'Bricklaying' and 'Roofing', saying that

• 'Bricklaying' most finish no sooner that 10 days before 'Roofing' starts.
• 'Bricklaying' most finish no later than 5 days after 'Roofing' starts.

When dragging the project along the plan, the relative constraints are respected, ie when 'Bricklaying' meets an obstacle it will rest until the span between it and 'Roofing' reaches 10 working days, at which point it will start moving again.

The video Moving a project illustrates the point.

For a more complex example see Home Construction Co.