A cello is a large, stringed musical instrument. A cellist has to sit in a chair to play a cello.
One end of a cello rests on the floor as it's played, with the neck upright. The cellist forms notes with her fingers on the strings and pulls a bow across the body of the cello. Cellos are an important part of an orchestra, though they're often featured in contemporary popular music as well. The word is short for the Italian violincello, which ultimately comes from the Medieval Latin vitula, "stringed instrument."