
When a manga work is "licensed", that means that an American publishing company (like Viz or Tokyopop) has actually paid the Japanese artists and publishers money in exchange for the right to publish a particular manga in the U.S. Subsequent sales of the newly Western-released manga also funnel back royalties to the original artist.
Yeah... thats the best way I could explain it...
Hope this helps!
-Mak0t0

To expand on this: the artist who creates the work holds the copyright to the work. It belongs to her. She sells publication rights to the Japanese publisher who may also purchase foreign sales rights. The publisher then makes copies available for readers to purchase. A percentage of each sale goes back to the artist. This is called a royalty.
The Japanese publisher can then sell foreign rights to foreign publishers. The artist will usually receive a percentage of this payment.
Scanlations on this site and sites like it are illegal because there has been no agreement between the artist and the site to share the work and no payment has or ever will change hands. The operators of this site make money through the pop up ads everyone keeps complaining about. None of the money goes to any of the artists of the works you are enjoying. It goes to the owners of the site. Scanlators, the site owners, the readers are all stealing from the artist.
Now, why licensing impacts you as a reader on this site: Apparently, the Japanese publishers don't police these sites, but US publishers who buy rights to publish these works in English do (and they have every right to do so) and so when a work is licensed by a US publisher, some scanlations groups are scared off and stop posting scanlations because they could potentially be sued because what they are doing is illegal.
Yes, it sucks for those who can't buy manga in their countries. Yes it sucks for those who can't afford to buy manga. Yes it sucks to wait for the US volumes. What would be nice would be if the Japanese publishers actually worked with the scanlators and provided a subscription service to readers, like Netflix for manga. I would totally pay a monthly fee to help the mangaka and get translated volumes faster.
What does it mean when a manga get licensed? Why can't it be scanned anymore?