A jeremiad is a long written composition with very mournful or dire overtones. This term is often used in a pejorative sense, to imply that a piece of writing is overwrought and overblown. If someone suggests that a piece of writing is a jeremiad, they usually mean that it could benefit from some judicious editing to tone down the nature of the piece and trim out extraneous content.
This term is a reference to the Old Testament prophet Jeremiah, who was famously long-winded. In the sense of a piece of writing with dire overtones, the word references the warnings of Jeremiah about the destruction of Jerusalem, which he suggested was the result of ignoring God. Mournful works are also called jeremiads in a reference to Jeremiah's lamentations about the destruction of Jerusalem, which happened just as he had predicted it.
Jeremiads in the sense of warnings commonly incorporate charged, polemic language. For example, many political authors write dire warnings about the future of their nations, referencing dissolute habits, unsound policy, and a general recklessness among the people which they believe will lead to dire consequences. Many jeremiads include a certain amount of railing against wicked or evil habits, with the suggestion that these habits be amended before a nation or a people suffers downfall.
People don't just use this term to describe a collection of dire predictions and warnings. A jeremiad can also be a reflective piece, referencing Jeremiah's lamentation. If a reflective piece is extremely mournful and it seems more like a long complaint than a serious reflective assessment, it runs the risk of being classed as a jeremiad. Especially when people speak or write about issues of great personal importance, it can be difficult to avoid intense emotions, even when such emotions dilute the power of the piece; in these situations, a good editor is vitally important.
You may also hear authors attempt to refute the idea that a piece is a jeremiad, typically in the introduction. These authors are generally aware that their writing style and the topic could potentially combine to create a piece which could be viewed as a bit overblown, so they attempt to convince readers before they even begin that the piece should be taken seriously. Sometimes this ploy backfires, making readers even more aware of the emotional overtones, and therefore even more critical.