This means that China has a trade surplus and a positive trade balance.
In the economy, trade relations are classified between exports and imports. For countries, it is interesting to export more than import, as this means that more money enters the country than it exits.
In this context, if China sells more than it buys, it means that China's balance of trade (exports minus imports) is positive, which is to say that China has a trade surplus.
Note: The difference between export and import is called surplus, when it is positive and deficit, when it is negative.