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(みじか)い」は長(なが)さがない時(とき)に使(つか)います。例(れい)：スカートが短(みじか)いです。