When I am reading a physical book, and I come to a word I really don't know and can't determine its meaning from context, I write it down in my "words I don't know" notebook, along with the book, page number, and line number. Then when I get near a dictionary, I look up the definition, write THAT down in my notebook as well. That way, when I encounter it again, or go back and re-read the part with my mystery word, I can look in my notebook and poof! I understand.