Radical
A semantic component of a Chinese character — the building block dictionaries are organised by.
A radical (部首, bùshǒu) is a recurring component used to build Chinese characters and to organise them in dictionaries. The standard mainland radical list contains 214 entries — the same set used since the Kangxi Dictionary of 1716, lightly updated for simplified characters.
Most characters have one radical that signals meaning and one or more phonetic components that hint at sound. The water radical 氵 appears in 河 (“river”), 海 (“sea”), 湖 (“lake”), 流 (“flow”) — every one a water-related word. The 氵 doesn’t tell you the pronunciation, but it narrows the meaning enormously.
Learning radicals isn’t strictly required to read Chinese, but it makes new characters far less random. Bookverse highlights radicals when introducing new characters, so the second time you see 氵 you have a foothold.