A species' niche is its ecological role or "way of life," which is defined by the full set of conditions, resources, and interactions it needs (or can make use of).
Each species fits into an ecological community in its own special way and has its own tolerable ranges for many environmental factors. For example, a fish species' niche might be defined partly by ranges of salinity (saltiness), pH (acidity), and temperature it can tolerate, as well as the types of food it can eat.
There are two main components of an organism's niche:
the set of resources an organism needs or can utilize (what it eats, where it nests, what time it hunts, etc)
the organism's role or position in a community (what eats it, how it cycles matter in the ecosystem, how it impacts other organisms)