Oftentimes when I'm reading a book there are words I don't understand.
Usually I just gloss over them, because I have heard them before or read them before. The thing is I don't stop to look them up because I kind of assume I know what they mean. The funny thing is though, from time to time, when I do look them up I'm usually very wrong.
This assumption of thinking I know a word can throw off an entire passage of prose. But that's just because I think I know what's up. The alternative, though, is to stop the flow of reading, pull out a dictionary (or just an app on my phone) and search for the word. If it's a complex book this might happen a couple of times a page which is very annoying!
So, what's there to do?
Well, I have started to jot down words I didn't know in a little book and keep it nearby. This method works well because I can continually come back and refresh my memory - effectively repeating the word enough so that it sticks forever.
App's help too, and the benefit there is that you get example sentences to reinforce the context of the word.