Usually air travels from our windpipes down into our lungs through small tubes. The tubes that come off the windpipe are called bronchi; then they split into bronchioles which are a bit smaller. Finally the small bronchioles split into tiny tubes called alveoli. In a normal person all these tubes are fairly small: up to 2 cm in diameter at the windpipe, all the way down to just a few millimetres for the alveoli. Our airways are a bit like a tree: the tree trunk is the biggest bit, but it gradually splits off into branches, then twigs.
This is a companion discussion topic for the original article at https://patient.info/chest-lungs/bronchiectasis-leaflet