Nematocysts:

A specialized cell in the tentacle of a jellyfish or other coelenterate; containing a barbed or venomous coiled thread that can be projected in self-defense or to capture prey.

What are they in the food chain?

Cnidarians are the consumers of the food chain because they DO NOT produce their own food.

And

Nematocysts

How nematocysts works

Sea turtles, one of the Cnidarians predators

What they are.