44 is the country code for the UK.

The bit before that is whatever you dial in the country you're in to get access to international numbers.

So if you're in France, you start with 00 before the 44 to reach a UK number.

But if you're in Australia, you have to dial 0011 before the 44.

And so on.

Mobile phone networks make it easy by allowing you to use the + symbol immediately before the country code, so dialling +44 ... from a mobile phone in any country should get you access to the correct number.