An organism’s ecological niche is, effectively, a resource that only that organism capitlizes on. Maybe it’s a type of plant only it eats (like the Koala Bear, who eats Eucalyptis), maybe it lives somewhere nothing else can survive (like a cactus living in the desert), or maybe it interacts symbiotically with another creature (the birds that eat bugs off of large animals).
Adaptive Radition refers to species constantly branching out and evolving in different paths. Physical shape, size, diet, temperature tolerences, etc all change and evolve. Adaptive radiation is the mechanism for organisms finding an ecological niche; the ones that do survive, the ones that don’t are selected out.