Basically, chajougu means "tea funnel" or "tea funnels". The chajougu set, which is mainly used in Chadou (Japanese tea ceremony), usually contains three items:
- 1 - A larger funnel for filling a natsume (container for thin-tea matcha)
2 - A smaller, more steeply sloped funnel for filling a chaire (container for thick-tea matcha)
3 - A cup on which the used funnel can be placed, to avoid spillage of any remaining matcha.

The above photograph is from an online shop in Japan (this particular set is made of cherry wood).
This is how you use them:

You'll notice that the actual shape of the matcha "heap" inside the container depends on the type and shape of the container itself.
At left, you make a slightly rounded heap for a large, medium-size, or small natsume.
At centre, you make a flatter, more rounded heap for a hira-natsume (a type of natsume that is short and wide - hira means "flat")
At right, you make a taller, pointed heap for a nakatsugi (a tall, thin type of natsume).
These funnels can help you make a perfectly-shaped mound of matcha. Because the guest usually examines the tea container after the tea is made (and in the case of natsume-s, most of the matcha still remains), it's important for the host to present a beautiful "landscape" of matcha.
Having said this ... my tea sensei prefers her students to fill the natsume or chaire slowly and painstakingly, one teascoop at a time. You can make just as perfect a matcha mound that way; it just takes more effort and attention!