It refers to a lung disease caused by inhaling very fine silica dust.
However, it's important to note that this word is considered a constructed word and is not commonly used in everyday language.
There are other languages that boast even longer words. For example, in Finnish there's the word "älyttömyydellisyys" which translates to "absurdity".
Ultimately, defining the "longest word" depends on how you define "word" and which language you're looking at.