You may have a lot of things to do AT WORK, but that is far away and tomorrow's problem. You may have some things to do AT YOUR PARENT'S HOUSE, but you won't be there for a couple of hours. When you have something "at hand", it is right there in front of you. It's not far away, and it's not in the future, it is in your hands, waiting to be dealt with.

"At hand" usually means nearby or close. It can be close in TIME as well as space. So, In your example, it means " needing to be dealt with now" Cambridge dicdtionary says: at hand happening or present at this time: The space shuttle crew must stay completely focused on the task at hand. https://dictionary.cambridge.org/us/dictionary/english/at-hand

It is a way of saying “Here” or “in front of you”. So the sentence would mean, “Concentrate all your thoughts on the work in front of you.”