usually it refers to policies which (usually unintendedly, sometimes explicitly) take from the productivity of younger people to finance older people. Because it is difficult to predict the future, the fear is that the unintended consequence is that this may take so much from the young that it robs their future. There are very good reasons to at least consider this as a possibility, because policymakers seem to make state revenue projections that are consistently overoptimistic. Also, programmes that were instituted as temporary often wind up becoming permanent and then also get mission creep; some programs that wind up being generational are instituted without taking into account demographic shifts, especially in fertility rates, etc.)

The lost decade in Japan is often cited as such an example, but increasingly we are seeing things like wage stagnation among <30 in the US and Europe that may also be the result of this (of course plenty of mainstream explanations abound as well).