Odds are an expression of ratios or likelihood depending on the grammatical context in which they're expressed. They may have multiple contexts in some cases. They may or may not be the direct subject of the statement in which they're expressed. Odds regarding likelihood are used to define betting prospects in gambling.
Examples of Odds:
Odds as ratios: This is a simple comparison of two elements, like 1:1, 1:5, etc.|
Odds as likelihood: This is the gambling context in some cases. A horse is backed at odd of 100/1, meaning payment of 100 times the stake.
Odds as subjects: The odds were severely against getting the job done on time.
Odds as descriptors: The besieged soldiers were facing odds of 20 to 1 at this point in the battle.