You've probably had the experience of being absorbed in some particular task or problem, so that your attention is focused, narrowed-in on one particular object or activity. If someone tries to talk to you at a time like this you don't really hear what they say. A focused attention excludes much of what is going on around you.
Sometimes being absorbed in this way feels good. When you are "in the zone" playing a sport, or captivated by music. You lose your sense of self. The digressive, churning activity of thought slows down or vanishes completely.
Sometimes being absorbed in this way does not feel good. When you cannot stop ruminating about a problem, or when your fears won't leave you alone. You feel anxious, trapped. You cannot enjoy your food or the company of friends because your attention will not slacken its rigid grip on the problem.
Probably you have had the experience of being tired and sluggish, bored and listless. In this mood attention fails to fix anywhere. You cannot focus enough to read a book or watch television, you barely take in what is said to you. Thoughts ramble and churn but go nowhere, they are never really about anything. You feel trapped in a fog.
And possibly, you have had the experience of being completely open, when your attention is not focused on one particular object, and the whole world lies before you all at once. When the colours and tastes, even the touch of a breeze on your skin, all have a depth and richness not usually experienced, yet your awareness is free to move to whatever calls it, not getting fixed anywhere.
Attention functions in different ways. And there is the question of how it moves from one way of functioning to another. You can deliberately fix your attention on something, or it can be captured against your will. Sometimes concentration comes easy, sometimes it is a struggle. Sometimes you can relax your concentration; at other times, you can't.
We look, too often, to the world around us in order to know how we are. We focus on our external circumstances, and on the opinions of other people. Our attention is itself the nature of this looking, the functioning of this awareness. But attention itself gets little attention. It is taken for granted.
Meditation means to stop taking this for granted. It means to turn your curiosity inwards, toward the functioning of attention itself.
In formal meditation sessions, you can experience attention in all its different moods. I particularly love to notice the way the attention can change. At the end of a group meditation sit someone rings a bell and it seems that, in a split second, attention undergoes a change. The mind that was lost in thought suddenly shifts to the sound of the bell; the mind that is held in a rigid focus suddenly relaxes with relief, like air escaping from a balloon. There is awareness before the bell, and there is awareness when the bell sounds, and there is awareness after the bell; but attention undergoes a transformation as the mind announces "ah, now the meditation is over".
You have probably heard the simile which says that the mind is like a wild monkey, swinging here and there, and the whole point of meditation is to tame the monkey, to make it calm down. To focus attention and become one-pointed.
Personally, I think monkeys belong in the wild. I do not think they should be tamed and domesticated. It is easy to have ideas about how the mind should be. But we should at least try to understand this thing called attention, and observe directly how it behaves, how it moves and flows.