Consider the function of a wooden mouse trap: to catch mice. The trap has several components which are all necessary, working together, for the trap to function.
How many mice would you catch with just the wooden base, or with just the spring, or just the bait? What advantage do you gain in eradicating the mice if the trap doesn’t work?
The mouse trap is irreducibly complex because you can’t reduce the components (i.e. do without any of the parts) and still have the complete function. Said another way, the trap’s complexity (total of the components) must remain intact to manifest its function.