Bookmakers quote fractions, exchanges quote decimals, and the number you actually want to reason about is usually neither.
The three forms
Fractional — 5/1 means five units of profit for every one staked. Add the stake back and you have the decimal.
Decimal — the total return per unit staked, stake included. 6.0 returns six for every one. This is the form worth using for anything involving arithmetic, because you can multiply by it directly.
Implied probability — one divided by the decimal odds. At 6.0 that is 16.67%.
Implied probability is not the real probability
A bookmaker's prices add up to more than 100% across a market. That surplus is the margin, and it is how the book makes money.
Add up the implied probabilities of every selection in a market and you will get something like 105%. The 5% above 100 is the overround. It means the implied probability of any single selection is always a little higher than the chance the bookmaker actually assigns it — the price has been shaded in their favour.
This matters when comparing prices between bookmakers: a market with a lower overround is offering better value across the board, regardless of which selection you fancy.