From Hard Drop - Tetris Wiki
ARE, also called entry delay, appearance delay, or spawn delay, is the period of time between the lockdown of one tetromino and the appearance of the next tetromino. ARE is a feature often seen in Japanese games, most of them influenced by Sega's early 1988 arcade version. Depending on the level, the amount of ARE may shorten. Some but not all analyses generalize ARE to include the extra delay after a line clear.
 During ARE
Generally, during ARE, the player must wait for the next tetromino to appear, and cannot actively cause anything to happen. However, ARE can serve 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 tetromino appears. Certain games like DTET also contain mechanisms to actively skip portions of ARE, speeding up gameplay.
 Word origin
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 "ah 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.
 Presence or absence of ARE
ARE is a vital gameplay factor in TGM, as its existence allows for many systems like DAS preloading, IHS, and IRS to exist. They do not exist, however, in popular American and European versions such as Tetrinet and most Tetris Guideline-compliant games, except possibly after a line clear. It is debated whether the lack of ARE is a requirement in the World guidelines or not; TGM3 (Ti) and TGMA do have ARE even though they seem to follow the guidelines. An oddball is iPod Tetris, which spawns the piece immediately after lockdown, yet does not accept player input until a split second later.