What does plus mean at the start of a telephone number?

+1 is the telephone country code for North America (USA, Canada, and certain nearby islands). When displaying a telephone number in international format, put the country code (including the plus sign), followed by the city code or area code (if any) and the subscriber number, using only blank spaces to separate groups of digits.

For example, (314) 555-0123 would be written as +1 314 555 0123

(The plus sign means "insert your international access prefix here." From a GSM mobile phone, you can enter the number in full international format, starting with the plus sign. The most common prefix is 00, but North America (USA, Canada, etc.) uses 011, Japan uses 010, Australia uses 0011, and many other countries use different prefixes.)