When you sign in to diynot (or come to diynot, if you have auto sign-in), certain topics / threads / sub-sections of the forum are marked in orange. These have had a reply since your last visit! Clicking that button will mark everything as white, as though you've read all the new topics / posts, or closed the forum and come back.
You may also notice certain threads are a slightly paler blue. This means you have already clicked that link before, it may have been yesterday or last week. A dark blue title means you haven't read it, at least not in the last week or so anyway.