中流 literally means 'midstream' — the middle of a river — but it's more commonly used metaphorically to mean 'average' or 'middle level' in quality, ability, or social status. You'll see it in phrases like 中流社会 ('middle-class society') or 中流水平 ('average level'). It can also appear in the chengyu 中流砥柱 ('mainstay, pillar'), where it keeps the literal river sense.