Ohh, how fun! Personally, I don't have anything underlined now (because me and reading don't mix lol ), but I did used to do this when I was younger and read the "Warrior Cats" series.
I'd basically underline a line or two when I thought it was important to the plot mostly because I didn't used to read for long periods of time. So, in case I took a break and came back, I'd just skim though the lines I underlined/highlighted and pray I get a recap to the story up to where I left my bookmark at haha.