Most commonly used with a tendu, dégagé, or grand battement, en cloche refers to the movement of the leg swinging from front to back (or back to front) without complimentary involvement in the upper body. (En balançoire represents a similar movement but includes complimentary movement in the upper body)
The concept of en cloche is used frequently in barre work, but knowing the origins can provide a great teaching tool for you since the movement mimics so closely imagery of the object.
Cloche is the French word for bell. Imagine a hand bell sitting in place on the floor. The dancer's head and torso make up the handle of the bell, and an imaginary bell envelops the body of the dancer from the waist down. The dancer's working leg serves as the bell's clapper, swinging front and back to ring the bell.
En cloche - pronounced [klɔʃ] "en" sounds like the English word on but trying not to pronounce the final n, "cl" sounds like the cl in "clap," "o" sound like the o of the word long in English except that it is pronounced with an open mouth instead of rounded lips, and "che" sounds like the shhhh used to shush someone.
In an interesting side note, à cloche-pied is a French expression meaning "on one foot." Sauter à cloche-pied would be to jump up and down on the same foot. The expression has existed since as early as the 14th century and could certainly be related to the origins of en cloche as well.