Tombé means fallen (from tomber meaning to fall) and the movement itself can be traced back to the Middle Ages. Both the term and definition fit the movement perfectly as the dancer’s weight “falls” toward the working leg and lands in a plié, giving both the change of weight and lowering of the overall profile in the plié.
Tombé traces back to a dance called a gaillarde, which was a social dance of the Middle Ages. The gaillarde was a lively dance that consisted of quick small jumps that stayed close to the floor. As gaillard often means bawdy or lively in modern French, one can imagine that this dance was a favorite.
Tombé is pronounced [tɔ̃be]. As in English, T sounds like the initial T in the word tart, tangy, etc., -om combined makes a nasal vowel in French, meaning that the m is not clearly pronounced and the o sound is produced by the combination of the mouth and nose, sounding close to the o in the word “on” but without a clearly pronounced consonant to follow it. -bé sounds similar to the word “bay” (without the final y).
Tombé in its earliest incarnations is always described as commencing with a rise on both feet or on the supporting foot, from here the dancer extends a leg in any direction and shifts the weight onto the working leg, landing in a plié.
Tombé is often used at the barre with the dancer returning to the original supporting leg followed the tombé. In other words, the weight shifts forward onto the working leg and then back to the supporting leg. Tombé is also used as a traveling movement. The dancer extends a leg in any direction, executes the tombé, and moves toward the working leg. It is often seen in the combination: tombé, pas de bourrée.
Here's a video to a modern interpretation of the gaillarde: