Sure bet happen when odds accepted by the different bookmakers will allow you to have a guaranteed winning with any outcome of the sports match.
Suppose that you have to bet for a soccer match on home/draw/away outcomes, dealing with different bookmakers. Let the best oddsfor this match on home/draw/away outcomes are 2.80 /3.50 /3.10.
The odd 2.80 means that a bet for 1 EUR gives you 2.80 EUR in case of a home winning. Hence if you will divide 1 EUR by this odd, 1 EUR / 2.80, then you will receive 0.36 EUR in case of a draw or an away outcome. Doing such with the other odds and adding the results, you get the cost of a 1 EUR payment irrespective of the outcome of the match: 0.36 + 0.29 + 0.32 = 0.97.
You will need 0.97 EUR to be sure to get 1 EUR back. That means if you would bet a 0.36 EUR in one bookmaker's office for the home winning, 0.29 EUR in another for the draw result, and 0.32 EUR in the third bookmaker's office for the away winning, you should be sure that your income will be a 1 EUR.
Off coure you need a calculator to do so!
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Notes: What you definitely need to be aware of, if you intend to play Sure bets, are the rules of the individual bookmakers. Get yourself informed about the costs of payment and pay-off. There are some other things to consider, when you play Sure bets — but if you do it, you have a nice possibility to upgrade your income.