What do the odds mean in Horse racing?

Traditionally odds is a measure of the likelihood OS something occurring. However in horse racing it measures what your payoff will be if your bet wins.
Fractional odds are what most of us are familiar with. If a horse is quoted at 4/1 then for every dollar you bet you win four dollars.
Decimal odds indicate what your payoff will be if your bet wins, including your bet. For example if a horse is quoted at 5.00 then you will be paid five dollars for every dollar bet. In decimal odds, a price of 5.00 is the same as the fractional odds of 4/1.
Moneyline odds indicate what your payoff will be in relation to a $100 bet. For example if the odds are 4/1 then the moneyline odds will be displayed as $400. If the odds are 1/4 then the moneyline odds will be displayed as negative $400.
Fractional odds 4/1 = Decimal odds 5.00 = Moneyline odds $400
Fractional odds 1/4 = Decimal odds 1.25 = Moneyline ofdds -$400