Answer:
A lethal allele results in actual fatality if it is the dominant or only allele in an organism's genotype.
Most lethal alleles are recessive, however, so they are rarely expressed in an organism and get masked by a dominant allele, resulting in the nonlethality of that gene. (i.e. something else covers them up, so the gene isn't lethal and the organism suvives). Hence, such an allele can be passed on for many generations before actually being expressed in offspring.