。
My father rarely takes extreme action.
Literal
Father [topic-は] rarely extreme action [contrast-は] does-not-take.
めったに~ない means 'rarely/seldom' — one of the standard frequency adverbs that require a negative predicate. 極端な is a na-adjective meaning 'extreme.' 行動をとる means 'to take action'; the contrastive は emphasizes that extreme actions specifically are what he avoids.