What does it mean for the UK to have an 'unwritten constitution'?

Unlike most other states, the UK does not have a single constitutional document. However, it can be said to have an 'unwritten constitution' based on the concepts of 'parliamentary sovereignty' and 'the rule of law'- a mixture of written laws and unwritten conventions. Parliamentary sovereignty is the idea that parliament is supreme - it can make or unmake any law. Like any other constitution, it serves to describe how the relationship between an individual and the state, and between the different branches of government, should function.