noun
red sash
General term for a red sash or obi, often worn with traditional clothing or as part of a uniform.
彼女は着物に赤帯を締めていた。
She wore a red sash with her kimono.
noun
red belt
Specifically a red belt in martial arts, indicating a certain rank or level.
彼は空手で赤帯を取った。
He earned a red belt in karate.
Black belt, a higher rank in many martial arts, contrasting with the red belt which may be an intermediate or advanced rank depending on the style.
Compound of 赤 (red) and 帯 (sash/belt). The martial arts sense is a direct extension of the literal meaning.