The intuitive answer to me would be to "emphasize" something. This explanation seems different from others I've seen, however, that say it means to "finish something". Help on this?
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.
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.