Have you ever lifted your head from your phone and realised that you detached from the real world for a moment?
Ever been so immersed in your thoughts that you suddenly "see" again, even though you were looking around the whole time?
My first true encounter with the concept of being present was in the book "The Power Of Now" - which I still consider the most influential book that I've read.
To be present is to detach from your mind and re-connect with your body in the world around you.
Your mind is constantly thinking about the past and the future, so when you switch it off for a moment you can exist in the present moment.
In a state of presence, I often laugh and think "We're all so wrapped up in our thoughts, in progress, in careers, that we never stop to look and enjoy what's around us."
The Power Of Now uses the example of when you're watching a sunset, and as the sun drops over the horizon, even if just for a moment, all that matters is the present moment - your mind switches off.
It's easy to find presence in moments of beauty like this... but much more difficult in day-to-day life - and it requires practice.
Meditation is a great way to learn how to detach from your thoughts. At the end of every meditation practice, I would recommend opening your eyes and trying to see how long you can "not think".
The moment you catch yourself thinking about something, bring yourself back to the present moment.
Deeply look at the room around you or out the window. Intensely listen to the surrounding noise.
Every time you catch yourself and return to the present it's like training a muscle.
This doesn't have to be limited to your meditation practice. If you find a spare moment, in a queue, or a waiting room, or when you close your laptop after completing a task...
Instead of reaching for your phone, take a moment to just be.
I would finally recommend reading The Power Of Now, which walks you through the process better than I ever could.