The catalytic converter is bad in exhaust and needs replaced. It is highly recommended to replace the o2 sensor at the same time.
http://www.obd-codes.com/p0420 --- check this out. Might find some of the info helpful.
Overall probably the biggest mistake vehicles owners make when they have a P0420 code is to simply replace an oxygen sensor (H02S). It is important to do proper diagnosis so you're not wasting money replacing parts unnecessarily. (BEST TAKE AWAY FROM DD'S LINK ABOVE)
Replacing the catalytic converter and o2 sensor is from Honda's technical website due to code p0420.
Looking for a Used Civic in your area?
CarGurus has 87,487 nationwide Civic listings starting at $1,499.
Search Honda Civic Questions
Honda Civic Experts