I don't think there a single feeling that corresponds to "difficulty concentrating". If I'm very tired then I have difficulty concentrating, but that's very different than the feeling of my mind being distract by topic A when I want to think about topic B.
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.
To any future monthly posters of SQ threads, please remember to add the "stupid_questions" tag.