It is commonly acknowledged that mobile phones have become an important accessory.

commonly 是形容词 common 的副词形式，common 意思是"普通的、常见的"，加上 -ly 变成副词后表示"普遍地、通常地"。 acknowledged 是动词 acknowledge 的过去分词形式，acknowledge 意思是"承认、认可"，在这里与 is 构成被动语态，表示"被承认/被认可"的事实。