Large countries like China are able to make products cheaply. This means that smaller countries like Wales cannot compete to make the same products. Therefore the factories in Wales making goods don't make any money and will close. This results in the workers losing their jobs.
When we buy from China the goods need to be transported to Wales via air or sea. This causes pollution and is adding to climate change. If we bought local goods they wouldn't need to travel as far.
If we buy from China we are not buying from countries who we are 'friends' with, like countries in Europe, the Commonwealth or America.