It's a lung disease caused by inhaling very fine silica dust.
While it's the longest word in common usage, there are longer words that are technically "words" in the English language but are very rarely used:
* Floccinaucinihilipilification, with 29 letters, means "the act of estimating something as worthless."
* Antidisestablishmentarianism, with 28 letters, refers to opposition to the separation of church and state.
So, while these other words might be longer, pneumonoultramicroscopicsilicovolcanoconiosis is the longest word commonly used and recognized by most English speakers.