expression, godan verb (-ku)
preach the truth; expound the Dharma
A set phrase used in Buddhist contexts for teaching or explaining the Buddha's teachings. The verb 説く means to explain or preach, and 法 refers to the Dharma or truth.
僧侶が集まった人々に法を説いた。
The monk preached the Dharma to the gathered people.
Compound of 法 (Dharma, truth) and 説く (to explain, preach). The exact historical origin is uncertain, but it is a conventional Buddhist expression.