These clefs are used, mostly, by strings, especially viola and cello parts, sometimes by the French horn (see the Brahms horn trio). If you have to play one of these parts on the piano, say to fill in for a missing cellist, just relate everything back to the middle C indicated by the arrow, as the diagram shows. It gets easier with practice, like everything. Just when you've got the part figured out, though, the cellist shows usually up and you're back to playing familiar bass and treble clefs.