gentleman's agreement

(literary) they

gentleman

to make an agreement, to forge a pact

an agreement, an accord, a convention

past tense and perfective aspect of 結ぶ (musubu) [godan]

tie (a string, rope, sash, etc.) / connect / be bound (by love, friendship, etc.) / enter a contract, form an alliance / wrap up, wind up / conclude, end

to bear fruit

those (referring to things distant from the speaker)

The bethrothed are to tie the knot. The newlyweds' fingers are bound together by a red rope, symbolizing that their destinies are now bound forever.