Layout table and cells grid lines
When you draw a layout cell in a layout table, a light grid of lines appears, extending from the edges of any new layout cell out to the edges of the layout table that contains it. These lines help you align new cells with old cells, and help you visualize the underlying HTML tables structure.
Dreamweaver automatically snaps the edges of new cells into alignment with nearby edges of existing cells. (Layout cells cannot overlap.) Cell edges also automatically snap to the edges of the containing layout table if you draw a cell close to the edge of a table.
You can also use the Dreamweaver grid, which is set and doesnt change based on the placement of cells, to help you lay out your page (see Using a tracing image).