短い (mijikai) means "short." It's an い-adjective used for something with limited length, duration, or height. It's the opposite of "長い" (nagai). For example, "このスカートは短いです。" (Kono sukaato wa mijikai desu.) means "This skirt is short." It can refer to physical shortess or a brief period.

「短い」は長さがない時に使います。例：スカートが短いです。