She copied a lot of that data onto a CD and passed it to Julian Assange, who is the head of Wikileaks. Wikileaks publishes digital data it receives from whistle-blowers, both summarizing and analyzing the data itself. It also provided access to other journalists (Der Spiegel, The Guardian, etc.) in order to incentive scoops and increase the impact of the leaked material. Some of the material includes the "Afghan War Logs", which is essentially raw military data on the U.S. presence in Afghanistan, including estimates of civilians killed in raids. The data itself is available to the public as an SQL or CSV database.
One of the pieces of data was the so-called "collateral murder" video which shows a U.S. apache helicopter firing on unarmed civilians, including a Reuters journalist who died and two children who were injured. Reuters had filed a FOIA request (Freedom of Information Act) to get public access to the video, but was unsuccessful. Since Manning had access to database, and because digital data has a marginal cost of $0, he was able to copy the video onto the CD passed along to Assange, thus bypassing the stall tactics of the U.S. government in releasing the video.
Manning was caught after the FBI had apparently enlisted Adrian Lamo-- himself a former hacker (or I guess cracker to be exact) who apparently Manning trusted and chatted with over IRC.
Also-- and I admit this is a digression from actual "hacker news"-- the U.N. Special Rapporteur on torture publicly stated that the U.S.'s treatment of Manning while awaiting trial was "cruel, inhuman and degrading." In fact, when someone from the press finally got the nerve to ask about this, a U.S. state department official called Manning's treatment "ridiculous and counterproductive and stupid." (He then resigned.)
Anyway, we're talking about the release of a computer analyst who got caught confiding in a well-known hacker, and who was sent to prison for passing the largest collection of digital documents to someone who is at present probably the most well-known hacker in the world.
If this subject isn't relevant to a site called "Hacker News", what is?
