Decimal odds
Decimal odds express a price as the total return per unit staked, including the stake itself.
In plain English
A price of 2.50 returns 2.50 for every 1 staked, which is 1.50 profit plus the original 1 back. Because the stake is included, decimal odds are easier to convert to a probability than American odds.
Example
A 20 stake at 3.00 returns 60 in total, of which 40 is profit.
Why it matters
Decimal is the cleanest format for arithmetic, so it is worth converting into it whenever you want to compare two prices or compute an implied probability.