basically..it means anyone born in say 1900 is in the same generation as anyone else born within their lifetime, say 80 years later. overlapping means the 80 year old--in 1980 overlaps within anyone born within the lifetime of the one born in 1980...so 80 years on we get to 2060, when the next overlapping generation begins.
i think..but then i'm not a dub.
the above explanation is subject to change, revision, nulite or whatever buzzword escapes from the watchtowers rectum.