What does it mean to "draw a line under something"?
Solution 1:
The idiom does have a different meaning from "emphasize".
draw a line under something
if you draw a line under something, it is finished and you do not think about it again: Let's draw a line under the whole episode and try to continue our work in a more positive frame of mind.
Solution 2:
Think of it like writing down a column of figures to add. When it's time to finish, you "draw a line under it", do the arithmetic and move on to the next one.