I guess they mean to top-up the system after air has been removed.
A conventional boiler (roof tank) will top itself up. A combi boiler will have a filling loop, usually under the boiler. Sometimes a single tap, or two knobs up under the boiler housing.
Just check your user instructions
i guess now its so cold more people are feeling if their radiators are getting hot enough and finding there cooler at the top than bottom,hence the bleeding