"Niche" here means the thing, or set of things, that a person can do well, or even best, the metaphorical place where a person is well suited. This usage is apparently derived from the use of "niche" as a technical term in ecology, where it refers to both the specific habitat and life style of an organism -- what it eats, how it avoids predation, how it finds shelter, etc.
Merriam-Webster give for niche
a habitat supplying the factors necessary for the existence of an organism or species
: the ecological role of an organism in a community especially in regard to food consumption
as well as the more general sense 2a:
a place, employment, status, or activity for which a person or thing is best fitted
finally found her niche
Biology online gives:
(ecology) The specific area where an organism inhabits
(ecology) The role or function of an organism or species in an ecosystem
The Oxford living Dictionaries gives:
2 (one's niche) A comfortable or suitable position in life or employment.
‘he is now head chef at a leading law firm and feels he has found his niche’
2.1 Ecology A role taken by a type of organism within its community.
‘the niche left vacant by the disappearance of wolves’