The + (plus) symbol represents your international dialing code (also sometimes called the exit code), ie in North America to dial internationally you dial 011 then you dial the country code and phone number....to dial a UK number .... +44 20 XXXX XXXX you would dial 011 44 20 XXXX XXXX...
from the UK to dial north america 613-525-xxxx , you dial 00-1-613-525-XXXX
Since each country is different, it is abbreviated to a + symbol