Okay so first, A gene is the section of DNA on a chromosome which codes for the formation of a protein controlling a specific characteristic (or trait) of the organism.
The way a gene is able to regulate the presence of a trait, is through the dominance and recessive code of the gene.
As an example:
A mother has given birth to a child with blonde hair, with the gene code of the hair being Bb.
The mother has brown hair with the gene code of bb (brown)
The father of the child had blonde hair with the gene code of BB (blonde)
Your will get a 1/4 chance with the punnet square.