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

Many traits are inherited but not expressed in an organism, these are the recessive traits while those that get expressed are called dominant traits. Mendel explained that it is possible that a trait is inherited but not expressed in an organism by his experiment on monohybrid inheritance.

He crossed a tall plant (TT) with a dwarf plant (tt) and all tall plants were produced in the first progeny. But when these were now crossed amongst themselves, not all plants were found to be tall. Some were dwarf. (Inheritance of both traits in F2 progeny which were not observed in F1 progeny). Hence traits inherited may not always be expressed.