It can actually make sense in a "twist-the-scriptures" kind of way:
Many people have known their great grandparents. So, if you are one of those people who knew your great grandfather and your great grandfather was around 20 yrs. old in 1914, and your grandfather was born in 1924, and your father in 1954, and you in 1984, then there you have the overlapping generations.
Even though each one represents a personal generation, the overall generation of people that knew great grandpa (including you) is included in the "larger picture" generation. You will be influenced by that 1914 generation because you have had direct personal contact with those who saw the World War I (1914) generation arrive.
When the generation that had actual personal contact with the 1914 generation dies off (probably yours or your son's using the example above), then supposedly the end will have come. That over all "big picture" generation will not completely die off before the end arrives.
Is that explanation pretzel-ly enough?