taru-adjective, adverb taking the 'to' particle
brilliant; resplendent
Literary word describing something shining brilliantly or dazzlingly. Typically used as 燦と輝く or 燦たる.
夜空に星が燦と輝いていた。
The stars shone brilliantly in the night sky.
彼女は燦たる笑顔で迎えてくれた。
She greeted me with a radiant smile.
煌 (こう) also means 'brilliant' or 'glittering' and is used in similar literary contexts, but 燦 (さん) often emphasizes a softer, more resplendent shine.
輝く is the common verb for 'to shine' or 'to glitter', while 燦 is a bound literary form used adverbially.
The kanji 燦 combines 火 (fire) and 粲 (bright, clear), suggesting a brilliant, fiery light. The reading さん is the on'yomi. The word is used almost exclusively in classical or highly literary Japanese.