I constantly hear that the top CB’s shut down half the field, can you explain how that is so when to me (someone who doesn’t get the x and o’s) it just looks like they are covering one receiver. So I don’t get how one receiver taken out the game takes half the field

I think terms like these get a little too generalized when really shutting down half the field might be more applicable to some one like Richard Sherman than Darrelle Revis. 

Sherman tends to stay on his side of the field, while Revis (or Talib) might more often find themselves following a single receiver wherever he lines up. So Sherman might “shut down half the field” while Revis might “shut down the opposition’s top target”.

Both are somewhat hyperbolic, but the general idea is the same, that a cornerback is so good passes can’t be thrown at them.