expression
take it from me; I speak from experience; word to the wise
A set phrase used to preface advice or a warning based on one's own past experience. Often used when the speaker wants to emphasize that they know what they are talking about because they have been through it themselves.
経験者は語る、準備は早めにしておいたほうがいいよ。
Take it from me, you'd better prepare early.
経験者は語るけど、あの店はやめておいたほうがいい。
I speak from experience, you should avoid that shop.
経験者 is a noun meaning 'experienced person', while 経験者は語る is a full set phrase meaning 'an experienced person speaks' or idiomatically 'take it from me'.
A set phrase composed of 経験者 (experienced person) + は (topic marker) + 語る (to talk, to tell). The literal meaning is 'an experienced person tells', which naturally extends to the idiomatic usage of giving advice based on experience.