It's a genere in
japanese anime and manga that reffers to erotic but not abruptly sexual content. It can show full to
partial nudity, including nipples, cameltoes, panty flashing, grabbing boobs, but never exposed genitals, and is often displayed in a humoresque way. The term comes from the japanese pronunciation of the letter "H" (pronounced "eh-chi") as in Japan erotic content is rated "H" while in the rest of the world its more commonly rated "X".
Outside of Japan it's easier to understand Ecchi as
softcore, non-sexual and erotic and Hentai as hardcore pornography.