Let's say there is a race meeting at Wolverhampton in the UK, and the 1:25pm race has a horse named Red Gunner as the favourite at +275 (3.75).
The race starts in 20 minutes, and you place a $10 bet on Red Gunner now at its current price of +275.
In the minutes leading up to the race, a lot of money is placed on another horse, called Greengage, which was priced at +800 but is backed into +500 (6.00).
As a result of the betting activity just before the race, Red Gunner drifts out to +400 (5.00).
Red Gunner wins the race, and your $10 bet wins you $37.50 including your returned stake.
If you had placed a $10 bet on Red Gunner but chosen to back it at the starting price, you would have instead collected $50.
Of course, it can also work the other way. Red Gunner could have been backed into +175 (2.75) right before the race, meaning your winnings would have been lower than what you did end up winning.
Taking the starting price is taking a gamble on how the odds will change before a race, which could end up either increasing or decreasing your winnings.
It might be worth studying the odds for a few races for 10 minutes before they all start to get a feeling for how odds move, and how you can best use it to your advantage.
Also note that on some races, fixed odds may not be available.