What does odds in favor mean?

Saying that the odds of something occurring are a to b (or a:b) means that we expect the event to occur a times for every b times it does not occur. For instance, the odds of drawing tails on a coin flip are 1 to 1, so if we repeat the experiment a very large number of times, we will have 1 occurrence of "tails" for every occurrence of "heads". The odds of getting a 5 on a dice roll are 1:5, and so on.