That it's literally soldered to the motherboard, i.e. there are no DIMM slots to put in more or different sticks. I'm not sure if you can desolder and replace/upgrade them, but I don't think you can. Definitely not without special equipment though.
This is common practise in MacBooks (AFAIK) and ultrabooks, which is why it's typically recommended with laptops to buy the model with maximum amount of RAM you think you'll ever need on it (and can afford of course) in any use case you can imagine.