When a new mutation is introduced into a population, it occurs in only one individual, and it distinguishes this individual from all others in the population. The mutation may be passed down to further generations, in which case it distinguishes the descendants who carry the mutation from all other individuals. This type of mutation is known as a segregating mutation because it divides the population into two groups: those that carry the mutation and those that don't.
If a mutation is widely spread and eventually is found in every gene in a population, we say it has become fixed. It is no longer a segregating mutation because every individual carries the mutation in its genes. From that point on, the mutation is guaranteed to be passed on to all future individuals.
As an example, consider a gene labeled A. Originally every member of a population is of type AA. Now assume a mutation is introduced, so that one new offspring is of type Aa, where a is the mutated version of A. a is a segregating mutation as long as any individuals of type Aa or AA remain, but as soon as all individuals are of type aa then all future offspring will be aa.
The plot in figure 1 illustrates this process.
Figure 1: The spread of mutations in a fixed size population. Some mutations spread to several individuals but eventually die out (blue lines), others become fixed and part of every individual (red line).