The Sino-British joint declaration was signed by both the Chinese and the British about the ownership of Hong Kong. This was signed because, originally, the Chinese owned the island. However, after several fights between the two countries -which the British won- they claimed Hong Kong and kept it under British rule for over one hundred years.
However, in an attempt to make British and Chinese relationships better, Britain gave Hong Kong back to China under the condition that (until 2047) the citizens of Hong Kong would have a democracy and have freedom of speech. This was made official by the agreement on the 19th December 1984. Both the Chinese and British were expected to uphold the terms of the pact.
For example some of this terms were:
The name used for international relations will be 'Hong Kong, China'. In doing so it may maintain and develop economic and cultural relations and agreements with states, regions and relevant international organisations on its own and it may issue travel documents for Hong Kong,
It will have independent finances with its own budgets and final accounts, but reporting it to the Central People's Government. Additionally, the Central People's Government will not levy taxes on it.
The [HKSAR] will retain the status of a free port and a separate customs territory. It can continue the free trade policy, including free movement of goods and capital.
This shows that ,although China owned Hong Kong, the citizens of the Island still had the benefits of being a British colony. Therefore, this was the meaning of the Sino-British joint declaration.