Usually when you play at dice games a random number is generated. Let's suppose the website generates a number from 1 to 100 and you're playing with 50% win chance.

Generated number: 34

If you bet "high", you will win the bet if the generated number is above 50. If you bet "low", you will win the bet if the generated number is under 50.

So in this case, you would win the bet if you choose "low", because the generated number is less than 50.