How did Mendel explain that it is possible that a trait is inherited but not expressed in an organism?

How did Mendel explain that it is possible that a trait is inherited but not expressed in an organism?

Mendel made a cross of tall and dwarf plants. He saw that only one of these that is tall
trait appears in F1 generation, the dwarfness did not appear and it remained hidden in F1. But when F1 plants were selfed 2 kinds of plants were produced in F2 generation in 3 tall :1 dwarf ratio. It means dwarfness reappeared again in F1 generation.