What are constitutional conventions, their role, and effectiveness in the UK constitution?

‘I’m sorry to say that the prime minister’s decision not to recall parliament, and to engage in further military action in Syria last week, showed a flagrant disregard for this convention. It seems the convention established in 2003, and in the cabinet manual, is being tossed aside as simply being inconvenient.’

Former leader of the Labour Party Jeremy Corbyn criticized then Prime Minister Theresa May on her decision to launch air strikes in Syria in 2018 What are constitutional conventions, their role, and effectiveness in the UK constitution?