Refers to a situation where one gains more than one loses; a net positive outcome. Often used in the phrase 取り得しかない (nothing but gain).
This deal is nothing but gain.
彼の提案は取り得が大きい。
His proposal offers a large profit.
Compound of 取り (tori, continuative form of 取る 'to take') and 得 (doku, 'gain, advantage'). The reading どく is a rendaku form of とく.