ARE is the period of time between the lockdown of the previous piece and the appearance of the next piece. Depending on the level, the amount of ARE may shorten.
During ARE, the player must wait for the next piece to appear, and cannot actively cause anything to happen. However, ARE serves as a time for the player to input DAS, IRS and IHS actions which will be processed in the moment ARE finishes and the next piece appears.
ARE is a word of Japanese origin, and though it looks like an acronym, it does not stand for anything. It derives from the Japanese word, "あれ (are)," pronounced "a ray" quickly. The word "あれ" is a demonstrative pronoun, literally translating to "it" or "that". Japanese players originally referred to this in-between time using the word "あれ" as there was no good word to describe it. The capital letter alphabet form "ARE" (pronounced ay arr ee) became popular when someone wrote it in that form, possibly owing to its cooler, acronym-like look.
The Existence and Lack of ARE
ARE is a vital gameplay factor in TGM, as its existence allows for many systems like 20G, DAS, IRS and IHS to exist. They do not exist, however, in popular American versions such as Tetrinet and most Tetris Worlds-compliant games. It is debated whether the lack of ARE is a requirement in the World guidelines or not; TGM3 (Ti) and TGMA in particular, do have ARE even though they seem to follow the guidelines.