“Concentration” as a concept is rather vague, and in danger of being misinterpreted or of having meditation students bring their own preconceived ideas to it. I prefer to use the more accurate and useful term, “stable attention.” It’s more descriptive of what we’re actually trying to do in meditation.
Stable attention is the ability to intentionally direct and sustain the focus of attention, as well as to control the scope of attention. Intentionally directing and sustaining attention simply means that we learn to choose which object we’re going to attend to, and keep our attention continuously fixed on it.
Controlling the scope of attention means training the mind to adjust how wide or narrow our focus is, and being more selective and intentional about what is included and excluded. Again, as an analogy, consider how vision works. To see something in all its detail, we must hold our gaze steady for as long as necessary, while focusing neither too narrowly nor too broadly.