It's a mutual protection treaty, basically. It states that, if a country from outside South America attacks them, they are obligated to help each other against it. But it draws the distinction that this only applies to foreign (as in, from outside of South America) aggressors.

In addition, it's a general agreement to assist one another and help each other grow and prosper. 

Then they're like "If you are South American, you can join.", and state that they will hold biannual meetings.

In short, it's like a South American NATO, but with a slight economical aspect to it.