Meditation is many things. It can be used for many things. The most common form in use today is mindfulness meditation. It is being fully aware of the here-and-now, and not in other worries or thoughts.
Meditation aids in mental focus, which not only helps in a magic practice but also in daily life.
Another form of meditation is deep concentration, being single-minded about a specific thought or purpose. Again, this is great help with both magic use and non-magic use.
A daily practice of meditation trains the mind to do these things, in stead of struggling with keeping on a specific task or being 'in the moment.'
It quiets extraneous mental chatter, and has been shown to improve aspects of one's health.