What does Odds Against mean in betting?

Odds Against are prices that show an outcome is considered less likely. In most cases, the potential profit is greater than the original amount placed.

How should it be understood?

If an outcome looks less likely, the market offers a more generous price. In fractional format, that often appears as numbers such as 5/1 or 3/1. In meaning, odds against are the opposite of odds on: here the selection is not the favourite, but the riskier side, which is why the return is higher.