"Preach" has no single meaning, but is used to convey a casual sense of affirmation, acknowledgement, Said in agreement, something that is real, absolute, well said, a person dropping mad knowledge,
Or to indicate that something has impressed you favorably.
Its usage among young blacks has been parodied ad nauseam among clueless suburban whites.
Seamore .... speak..... preach…….
Man I just started a new fur line.... Preach