Wikipedia says: A single subspecies cannot be recognized independently: a species is either recognized as having no subspecies at all or at least two, including any that are extinct. I do not understand this concept, please explain.
Think about it like this:
How can a you subdivide 1 individual and still get a whole?
The answer is, you can't.
For there to be subspecies you need to have defining characteristics that can be used to separate individuals within the population into groups. This means that you need at least 2 to divide into.
Now there's a little tricky ground here - you could have an extinct subspecies as one of the subspecies (e.g. Homo sapiens neanderthalensis, if you follow the trend that Neanderthals are not a separate species) - meaning that the humans you see around you can't be subdivided into subspecies, but can still a subspecies that could be called Homo sapiens sapiens
In biological classification, the term subspecies refers to one of two or more populations of a species living in different subdivisions of the species' range and varying from one another by morphological characteristics. A single subspecies cannot be recognized independently: a species is either recognized as having no subspecies at all or at least two, including any that are extinct.
Thst is, subspicies are defined by opposition to each other: e.g., taller vs. shorter, bright vs. bleak, etc. Such a definition logically implies (at least) two types of specimen.