Attention is a kind of surplus mental capacity that we have, which isn't specialized, but can instead be directed to tasks as needed. Ironically, we also use the term for the dedicated mental system which directs this extra capacity, which makes talking about it a bit more complicated.

Most of the stuff we do, our brains just kinda handle for us. Walking is usually like that; it's an incredibly complex feat of dynamic balance, movement planning, and adaptation to changes in the environment, but it rarely takes any conscious effort on our part. Conscious effort is directed attention.