Just to be clear, this question relates to the 2008 film Journey to the Center of the Earth and not the original Jules Verne story.

In the scene referred to above, Trevor Anderson is on board a plane with his nephew Sean, headed for Iceland. Once there, they hope to investigate the remote volcano, where Max Anderson, Trevor's brother and Sean's father, met his doom ten years previously.

Trevor is deciphering some notations that Max made in his book. They consist of a bunch of letters arranged in pairs. For example "S-A", "I-S", and "G-G". There's also "P-B". Trevor seizes upon this example and says that there appears to be some kind of code behind the letters in Max's notebook that he thinks might have something to do with the periodic table. As even those of us with almost zero scientific knowledge know, Pb is the chemical symbol for lead. It's derived from the Latin word for lead, which is plumbum.

Trevor works out that "S-A" most probably stands for Sigurbjörn Ásgeirsson, who happens to be the executive director of the Ásgeirsson Institute for Progressive Volcanology. Trevor's certain that Max must have known this guy, so he determines that the Institute will be his and Sean's first port of call once they touch down in Iceland.