Why do some phone numbers have plus in front?

The + is used in international phone numbers. It indicates that the caller should dial the international access code for the country they are dialling from.

For most of Europe and Africa, this is 00. North American uses 011. Other countries may use other numbers.

For example, +44 20 7222 1234 is a number in London, U.K. It would be dialled as 00 44 20 7222 1234 from Germany, or as 011 44 20 7222 1234 from the USA.