沦 is a bound form (b.f.) — it cannot stand alone as a word and only appears in compound words. Its core meaning is 'to sink, fall into (a negative state), be submerged'. You'll see it in verbs like 沦陷 (be conquered / fall to enemy), 沦落 (be reduced to a worse state), 沦为 (degenerate into), and 沉沦 (sink into depravity). The older sense 'ripples or eddying water' (from the character's original reference to water rings) survives only in literary or classical contexts. As a HSK 7 bound form, you mainly encounter it in formal written Chinese.