a pump," your first impression is likely a metaphor, you will begin to change your mind and see the channels between bristles transport paint like a pump
When it comes to applying paints and other finishes, you have lots of brushes to choose from. Natural-bristle brushes (made with animal hairs) used to be considered the best money could buy, but some of today's synthetic (usually nylon) versions now perform as well. Choose a synthetic-bristle brush for latex and water-base finishes. Also consider inexpensive foam and disposable brushes if you paint only occasionally and want to save on cleanup time. Home Improvement Encyclopedia
Natural or synthetic bristle designed to hold and apply paint. Synthetic bristle brushes works best with latex and water-based paints because water is not absorbed as easily and causing the bristles to expand. Natural bristle brushes apply more smoothly.