The colloquial phrase "dollars to donuts", sometimes spelled "dollars to doughnuts" or used with other monetary units, indicates that something is exceedingly likely to happen. When placing a bet, the person gambling feels that their outcome is a sure thing. They feel so confident that they believe they could gamble their dollars against someone else's donuts, indicating that even though the bet is lopsided the gambler thinks there is no way they can lose and they'll end up with a box of free donuts.
Of course, many gamblers feel like they're going to win, which is why gambling exists. Sometimes this slang phrase is used to psyche themselves up to place the bet. The unfortunate problem is that sometimes the donuts win.