What does the longest English word mean?

The longest word in the English language is pneumonoultramicroscopicsilicovolcanoconiosis, which is a lung disease caused by inhaling very fine silica dust.

It's a mouthful, but it literally means "a lung disease caused by inhaling very fine silica dust."

While this word is the longest in the dictionary, there are longer words that can be formed using technical terms. However, these are not commonly used in everyday language.