Typically referred to as ageing it is a concept meant to communicate that organisms become less efficient and more vulnerable with age.
With the massive amount of processes required in the human body, and only ~20,000 genes in protein coding, each gene has multiple roles - they are pleiotropic. George C. Williams pointed out that pleiotropic effects pull in different directions and as far as a gene has early life benefits these are favoured over any late life negatives that may come along with them. Given its nature senescence begins after the age of reproduction (~18 in humans).
Cultural senescence manifests itself very similarly to that seen in human biology. An example would be our creation and distribution of technology which produces short term wealth and power but may harm us in the long run. The market favours short term gains and our system makes us comfortable now, but is liquidating the planetary resources at a rate which cannot continue. As well as this the impacts on human psychology over the long term are yet to truly be known.