People who have the same gene may be affected differently. Two terms explain these differences: penetrance and expressivity.
Penetrance refers to whether the gene is expressed or not. That is, it refers to how many people with the gene have the trait associated with the gene. Penetrance is complete (100%) if everyone with the gene has the trait. Penetrance is incomplete if only some people with the gene have the trait. For example, 50% penetrance means that only half the people with the gene have the trait.
Expressivity refers to how much the trait affects (or, is expressed in) a person. A trait may be very pronounced, barely noticeable, or in between. Various factors, including genetic makeup, exposure to harmful substances, other environmental influences, and age, can affect expressivity.
Both penetrance and expressivity can vary. People with the gene may or may not have the trait, and, in people with the trait, how the trait is expressed varies.