The answer to this question can be found in Chapter 17 of Guns, Germs, and Steel. Specifically, it can be found beginning on p. 347 of the paperback edition of the book. The most likely answer to the question is that the specialty of the Lapita people was pottery that was “tattooed.”
There was not, as far as we know, any group of people who referred to themselves as the Lapita. Instead, this is a term that archaeologists use to describe them. It is used because Lapita is the name of an archaeological site where their type of potter was first described.
Diamond says (on p. 347) that the Lapita people put designs on their pottery that might have been “inspired by geometric designs that they had already been using on their bark cloth and body tattoos.” This is why we say that their pottery was “tattooed.”
The Lapita people are important to history because they were apparently Austronesian people. Diamond discusses them because the appearance of Lapita pottery in the New Guinea area shows that Austronesians had reached that area. He then goes on to discuss how the competition between Austronesians and New Guinea natives turned out.