Cdma is the type of signal verizon's network runs on & they phones don't use sim cards like gsm (global system for mobile) carriers. The htc droid incredible 2 is a global roaming capable phone, so it has a sim card slot behind the battery, so when you go to foreign countries you can switch to global mode and still use the phone.
When you leave the store with your new dinc2 our get it in the mail it is set to global as preferred network, the phone is probably struggling to find a gsm network to connect to wasting battery power because it never finds one. Setting it to cdma stops this and allows the phone to operate normally on the signal it was primarily made to use. Pretty much all verizon sales reps don't know about this and will let you leave the store with a global cdma phone that is set to run on a gsm signal, (improperly set-up) and leave you to wonder why the battery life is so horrible.