ROI or "return on investment" means how much you get back for what you put out in wagers at the dog track or anywhere else. If you bet $2 and win and the payoff is $4, your simple return on that investment is $2. Of course, you don't win every bet.
When you factor in the money you spend on losing bets, you get your real rate of return. Let's say you make ten $2 bets on a program and hit 6 of them. For the sake of simplicity, let's say that each bet returns $4. That's an investment of $20 for a return of $24. You've made an ROI of $4.
Of course, this is just a very simple explanation. To be realistic, you have to figure in all of your expenses: programs, gas for your car, what you pay for food and drink at the track, any systems you buy etc, etc. It's plain to see that few people really know whether they're making a profit at the track or not. Do you?
The only way to figure that out is by keeping very good records of every bet you make and how much you win or lose on every bet. You can write it in your program and then transfer it to a little notebook or however you want to do it.
But if you don't do it, or something like it, you'll never know for sure how much you're winning or losing. Just make sure that you write the bets down right as you make them, instead of depending on memory. Memory can be a very tricky thing when money - and greyhound handicapping - are involved.