Let's start with flipping a coin. Obviously there is a 1/2 chance you'll land on heads.
But why?
Well it can only land on either heads or tails, which means that there are only two possible outcomes. You want to know the odds of getting one of those outcomes (in this case heads). Well it's 1 out of 2 or you can write that as 1/2
In a coin there are six possible outcomes. Rolling a three is only one outcome. So there is a 1 out of 6 chance of rolling a 3, or you can write that as 1/6