一会 is a measure word for a short period of time, like 'a moment' or 'a while'. You can use it after a number (usually 一) to indicate a brief duration, as in 等一会 ('wait a moment'). It is also commonly used as a time adverb meaning 'in a moment' or 'a moment ago', though that usage is more typical of the variant 一会儿. Note that the pinyin in this entry is yī huì, but standard tone sandhi would be yí huì because 会 is fourth tone.