The Colombia trade agreement would lock in that country's current duty-free access to the U.S. market for most of its goods and require Colombia to remove tariffs on U.S. exports and make other reforms to create a friendlier business environment for U.S. companies.
So it seems that for reasons that are unclear Colombia currently pays very little in tariffs to the US, but US companies exporting to Colombia pay a lot and this deal would set in stone the former and then reduce tariffs paid by US companies.
...okay, to the source (.pdf) Unsurprisingly this is about a lot more than tariffs. Knocks down a variety of barriers to US companies from operating in Colombia and exports our views of intellectual property, etc...
Anyway, this stuff is reported on at the level of "call it free trade so Tom Friedman will support it" but there's a lot more to these agreements and it'd be nice if journalists would actually inform people.