It's set up so if China or Canada makes an investment in each other's country and then the government changes it's laws and that law change causes the investment to become worthless Canada or China becomes liable for the loss.
Efeectively it prohibits China from expropriating Canadian companies who have invested in China. It also gives Chinese businesses protection here if they build a billion dollar gas plant in kitamat and the gas pipeline that has been approved were to be canceled.
There was a really good radio show on the CBC a few weeks ago discussing it called 180. I thought they provided a pretty balanced view of the agreement rather than the fear mongering. I am still concerned with the length of the agreement and the lack of opt outs.
The fear mongering is like when Nafta came into force people said that it would result in a water pipeline being built from the ice fields to California with nothing canada could do about it. It didn't happen.
Edit: here is the link to the CBC program.
http://www.cbc.ca/player/Radio/The+180/ID/2523881705/