godan verb (-su), transitive verb
shine on each other; illuminate one another
Describes two light sources or reflective surfaces casting light on each other. Often used in poetic or literary contexts.
月と湖が互いに相照らす夜だった。
It was a night when the moon and the lake shone on each other.
照らす means 'to shine on' or 'to illuminate' something else, without the mutual aspect of 相照らす.
Compound of prefix 相- (mutual) and verb 照らす (to shine on). The exact historical derivation is uncertain, but the meaning is transparent from its components.