遅い (osoi) means "slow" or "late." It's an い-adjective, the opposite of "早い" (hayai). It describes lack of speed (moving slowly) or being behind schedule (late for an appointment). For example, "電車が遅いです。" (Densha ga osoi desu.) means "The train is late."

「遅(おそ)い」は動(うご)きが遅(おそ)い、または時(じ)間(かん)が遅(おく)れる時(とき)に使(つか)います。例(れい)：電(でん)車(しゃ)が遅(おそ)い。