It's a maritime brokers' motto. Since 1801 the motto of the London Stock Exchange (in Latin "dictum meum pactum") where bargains are made with no exchange of documents and no written pledges being given.
"My word is my bond" as brokers say (or "my word, my bond!")
To do what you say and keep your promise. The Trump presidential campaign plagiarised a speech of Michelle Obama's in 2008 which used the phrase for Melania Trump to give at the RNC.
Michelle in 2008: "And Barack and I were raised with so many of the same values: that you workhard for what you want in life; that your word is your bond, that you do what you say you’re going to do..."
Melania in 2016: "From a young age, my parents impressed on me the values that you work hard for what you want in life; that your word is your bond and you do what you say and keep your promise..."