To not be where you are.
It means, your body is right here, but your thoughts are elsewhere in universe. They are in the past, somewhere between old memories or even replaying an argument that you still want to win for the hundredth time. Or in the possible future. Worried and stressed about what may happen at work, trying to plan for every possible scenario and through that hoping to fully control the future.
By spending the time to think about the past and the possible future, the more you - in my personal experience - tend to also be ineffective and miss life as it happens.
Overthinking zaps so much of your energy that you can lose motivation to take action. Additionally, if you are not fully here in this moment then it is very easy to miss and to not fully enjoy a victory or simply a beautiful, fun or small moment in life.
Of course, you cannot spend all of your time in the now because there are things you can learn from reexamining your past and there are things you sometimes need to plan for you in the future. But the kind of obsessive or addictive way to spend so much time in a regular week in the past of future can be replaced with something smarter and happiness friendly.
There are habits that helped me a lot in the past few weeks to make that shift. First, slow down. Start your day with doing whatever you do first in your morning slowly. This will help you to keep in slower pace and to focus fully on what you are doing for the rest of your morning. Do everything in calm and relaxed manner is often the quickest way to do something well. So you can get your mind back to what your body is doing.
Second, reminding yourself silently: Now I am....
And it could be that "I am brushing my teeth, taking a walk and listening to the sounds around me, etc. Just reminding your self to stop wandering and it brings your focus back.
Third, disrupt your thoughts and quickly reconnect with here and now. If you are dragged into much negativity, overthinking, use a stop-word or phrase to silence the inner critic. This works well. Just shout in your mind: STOP.
So hopefully it helps you to run your days ahead.
Take care :)