Airline is a flexible silicone hose to connect your air pump to your airstones or air-action ornament.  It is a standard size and is available in 3m or 6m lengths.  It keeps its shape even when coiled and doesn't kink like PVC hoses can.

Top Tip!

The longer the hose is, the more work your pump has to do to force the air down it, so always trim it as short as possible if you want maximum bubble performance!