Hit Range called Hit Judgement (攻撃判定 kōgeki hantei) or simply Judgement (判定 hantei) in the Japanese version, is a basic but ultimately fundamental gameplay mechanic in the Tekken series. Simply put, characters' attacks will have an associated range with each hit. There are 3 possible categories in which a hit can be classified (High, Mid, Low). Effective understanding of each categories' properties is key to winning.
"High" attacks (上段攻撃 j'ōdan k'ōgeki) are mainly strikes to the head area and can be guarded by a standing guard. High attacks can also be evaded simply by crouching. High attacks are rendered completely useless by a crouching opponent. They are also highly prone to attack reversals and parries. To counter-balance this, many high moves have very fast animations(low frame count). Most throws function as high moves.
"Mid" attacks (中段攻撃 ch'ūdan k'ōgeki) are strikes aimed around the mid-body and can be guarded with a standing guard but will disregard a crouching guard. Therefore, this is useful against crouching opponents. However, many mid-range attacks are given a frame disadvantage to compensate. This means they are potentially "unsafe"(easy to punish). Many launchers are mid-range and this is key to effectively using launchers.
There is another subcategory of mid-range attack. These are called special mids (特殊中段攻撃 tokushū ch'ūdan k'ōgeki) because they will hit an opponent on the ground but can be blocked with either a standing or crouching guard.
"Low" attacks (下段攻撃 gedan kōgeki) are strikes aimed at the leg area. These attacks will hit through a standing guard but can be blocked by a crouching guard and evaded simply by jumping. Low attacks are also susceptible to low parries.