Hi
The term lack of genetic diversity for a particular population refers to the fact that there are less number of alleles and less range of phenotypes and traits like (height, skin, color, ability to cope disease, ability to change metabolism as per requirement etc).
If we talk about endangered species, they are the specie which are prone to be extinct in near future because their population is very less in number. There can be multiple reason of becoming endangered for a population such as over hunting etc.
So when we say that an endangered population is genetically less diverse, it means that there are further difficulties that hinder their recovery.
For example: If a calamity stucks, all organisms that are already low in number and also genetically less diverse, so they cannot cope with calamity and get extinct. However, if this population is genetically more diverse, some organisms may survive some may die, and those which survive can recover the population. That is why it becomes more difficult to recover an endangered specie if it is genetically less diverse. The more the genetic diversity, better it is for specie.
Hope it help!
