A project may be broken up into separate steps, or milestones. For example, if you are hired to write a novel, you might have milestones like this:
MILESTONE 1
Create and submit a detailed story outline, including character arcs
and plot steps. $_________
MILESTONE 2
Submit first 50% of Rough Draft ( or X # of words) $__________
MILESTONE 3
Submit completed Rough Draft of Novel $___________
MILESTONE 4
Review notes from client, and make revisions / polish draft $___________
>>> In example above, the total price of the work becomes the sum of the completed milestones. That is what the total bid comes to - the sum of these steps. They are good for the freelancer, because you are paid as you work. Good for the client because they get to see what you are doing, and to make notes / comments / corrections as you go. Also, they don't have to pay the whole thing all at once. It gives both sides more security that payments and work are both progressing appropriately.