Some traits that are inherited may not express themselves. Such hidden traits are known as recessive traits. Mendel explained this phenomenon with the help of monohybrid cross. In a monohybrid cross performed by Mendel, tall plant was crossed with a dwarf plant which produced all tall plants in ${\mathrm{F}}_1$ progeny.
However, when these ${\mathit{F}}_1$ tall plants were crossed with each other, both tall and dwarf pea plants were obtained in ${\mathit{F}}_2$ generation. Reappearance of the dwarf pea plants in ${\mathrm{F}}_2$ generation proves that the dwarf trait was inherited but not expressed in ${\mathit{F}}_1$ generation.
So, dwarfness traits of plants (pea plant) were not lost but are suppressed in the ${\mathit{F}}_1$ generation and thus reappeares in ${\mathrm{F}}_2$ generation.