Attention has no independent value and only accompanies other processes. As a psychical process, it features its own particular physiological mechanisms. Figuratively speaking, attention is like a filter or a magnifying glass, which allows your brain first perceive some information, then gradually obtain more of it and ignore the rest of it.
The most understandable and well-known attention concentration mechanism is the orientation reflex. It is humans' and animals' inborn ability to respond to environmental changes. The strength of an organism's response depends on the signal's novelty. For example: somebody enters the room and you look in their direction straight away, even if you are busy with something. However, if you know that someone is about to enter while you are occupied, you probably won't get distracted. Nevertheless, if someone enters singing out loud, he will still attract your attention, because of his unusual way of appearance.
The energy of the brain's cognitive activity is wholesome. Focusing on one object distracts you from the other objects. If you could have a look into your brainpan, and if the areas of cerebral hemispheres involved into the cognitive activity glowed, you would see some kind of a beacon in a dark background in the area involved in the observation.