A rootkit [ http://en.wikipedia.org/wiki/Rootkit ] tells the OS [Windows, Linux etc.] to ignore certain files/folders so they become invisible, & thus can't be found to remove them. This sort of malware often does other things so that its operation & effects aren't detected, but it's the making files invisible part that justifies the Rootkit name. In this example the rootkit was designed so that when someone visited a hosted web site, it stuck it's own code into the stream of data sent to their PC/laptop web browser -- however just like on or with Windows, malware can be designed & written to do any number of things, e.g. record user input &/or find & forward files.
IMHO why it matters...
As the author of the 1st linked blog notes: "... it is interesting to see the cyber-crime-oriented developers, who have partially shown great skill at developing Windows rootkits, move into the Linux rootkit direction." The author at the 2nd link writes: "This rootkit, though it's still in the development stage, shows a new approach to the drive-by download schema and we can certainly expect more such malware in the future. "
More sophisticated malware infecting *nix servers means more servers will be infected, increasing the odds of you becoming a victim when you visit whatever web sites. *Nix rootkits would logically expand to capturing user info [if they're not there already], including payment data if/when possible. Malware & security software play a cat & mouse game, where malware developers come up with something new, followed by security software adapting to look for that too -- that [hopefully small] window, before security software adapts, can be more dangerous. It'll likely pay to be extra cautious when it comes to sites that poorly maintain their servers, whether through a lack of IT resources or through laziness, as these rootkits up the game.
http://blog.crowdstrike.com/2012/11/http-iframe-injecting-linux-rootkit.html
http://www.securelist.com/en/blog/208193935/New_64_bit_Linux_Rootkit_Doing_iFrame_Injections