the plus sign thing is the international standard way to write a number. the + represents the "international code"

for example in +16045551212 , the + means 'international' and the "1" is the country code for Canada

*Edit* another HF post:

Keep all your numbers in the international format and avoid the guessing game. GSM systems all over the world translate the plus sign in front of the number as the int'l dialing prefix and insert it when appropriate, i.e. only for calls going out of the given country.

Canada, USA and some other countries share the same international code (1) so all Canadian, American, etc. numbers should be stored as +1 area code number.

Okay, last *Edit* (HF again)....

The plus sign is part of a standard being adopted to describe an international telephone number. Basically the standard is:
-for a landline: plus sign ("+") then the country code then the area code then the telephone number.
-for a mobile: plus sign ("+") then the country code then the telephone number