In vacuum tubes the filament is usually a section of wire, often made of tungsten, that is connected to a supply voltage. Its purpose is to get hot and heat up the cathode, which excites the electrons and makes it easier to get them to jump from the cathode across the tube to the anode. Filaments get quite hot and will glow just like the burner in your oven. This is the source of the reddish orange glow you see in so many tubes. In some tubes the filament and cathode are the same element, but the basic idea is the same.