If the mutation occurs during meiosis, the mutation will be incorporated into a gamete. If that gamete is the one that eventually fuses with another gamete (i.e. if it's the sperm that fertilizes the egg), that mutation will be passed on to the offspring. As all the offspring's cells are the result of the first two gametes, all the organism's cells will have that mutation. Obviously this can have dire consequences for the offspring, if the mutation is harmful.