Constitutional monarchy is a form of government in which a king or queen acts as Head of State.
The ability to make and pass legislation resides with an elected Parliament, not with the Monarch.
As a system of government, constitutional monarchy separates the Head of State’s ceremonial and official duties from party politics.
A constitutional monarchy also provides stability, continuity and a national focus, as the Head of State remains the same even as governments change.
The Sovereign/Monarch governs according to the constitution - that is, according to rules, rather than according to his or her own free will. The United Kingdom does not have a written constitution which sets out the rights and duties of the Sovereign, they are established by conventions. These are non-statutory rules which can be just as binding as formal constitutional rules.
As a constitutional monarch, the Sovereign must remain politically neutral.
On almost all matters the Sovereign acts on the advice of ministers. However, the Sovereign retains an important political role as Head of State, formally appointing prime ministers, approving certain legislation and bestowing honours.
The Sovereign has other official roles to play such as Head of the Armed Forces.