po420

my 1996 ford bronco check engine light keeps coming on. I scanned the code and it says po420, catalyst system efficiency below threshold bank #1. I'm trying to understand but what does this mean ? i have one oxygen sensor on left and on right side, then a cat. followed by an oxygen sensor, followed by another cat. does this mean i should check to see if the sensor after the first cat is bad? or could the first cat be bad ?

Hey maybe this link will help:

http://www.findarticles.com/p/articles/mi_m0BQA/is_2_80/ai_71847178/pg_2