What is focus?

In order to concentrate on one thing you must, by default, ignore many other things.
Here's a better way to put it:
Focus can only occur when we have said yes to one option and no to all other options. In other words, elimination is a prerequisite for focus. As Tim Ferriss says, “What you don’t do determines what you can do.”
Of course, focus doesn't require a
permanent no, but it does require a
present no. You always have the option to do something else later, but in the
present moment focus requires that you only do one thing. Focus is the key to
productivity because saying no to every other option unlocks your ability to accomplish the one thing that is left.