How does it block out and absorb heat at the same time?
Seems like it was a poor choice of words. Trap would be more apt.
[How does it block out and absorb heat at the same time?]
When eneergy impinges on a surface it is either passed, blocked (absorbed) or reflected. Or a combination but all must be accounted for. It is that nasty conservation of energy thingy.
You can block something by either reflecting it away or absorbing it, think mirror sunglasses versus dark sunglasses. CO2 works like the dark sunglasses, preventing the escape of IR into space by absorbing the radiation and warming up.