Mendle's experiment showed patterns of inheritance in the laws of probility. If you have a die (one of a pair of dice) with four sides and each side has 1-4 dots, when you throw it, you have a1 in 4 chance it will be (lets say) 3.
So he says that a cross will be a chance of 1 in 4 of being a set of traits together. I am sure you have seen a Punnett square. Each mating will have a 1 in 4 chance of producing one of the crosses. He used tests that tested for one gene crossed with another. Perhaps color of corn. Each cross would have a chance of 1 in 4. So white crossed with yellow: The gametes would have either a W or a Y. When crossed, you could get WW, WY, YW or YY. If white is expressed over yellow, what you would see, would be 3 whites and one yellow.