Original: The term ecchi is applied to anime or manga that has vague sexual content (such as skimpy clothing, partial or full nudity), but does not show sexual intercourse, making it the manga equivalent of a PG-13 rated movie.
Simplified: The English use of this word is usually used in manga and anime and it is used when the anime or manga has sexual content, such as skimpy clothing or partial or full nudity, but does not show sexual intercourse.