The role of Parliment is to debate laws which typically are proposed by the executive and pass through several stages before gaining "Royal Ascent" and becoming law.
Parliament also has the important role of holding the executive, The Prime Minister of the United Kingdom and his ministers ([The Cabinet]?, to account. This is usually done through Parlimentry Questions and a Committee system.
The name is also used for the buildings in which both houses meet, which are officially called the Palace of Westminster.