Some embargoes are stronger than others: for example, a title might be available as a galley or ARC only to those industry professionals who have signed non-disclosure agreements, legally binding contracts prohibiting readers from discussing the contents of the book to anyone who hasn't also signed the agreement. Others are so colossally secret that galleys and ARCs are never produced, and virtually no one knows the contents of the book until the on-sale date. (An example of the latter would be the later Harry Potter novels.)
Now, embargoes are routinely broken, and I've actually never heard of a book that made it all the way to the on-sale date without having something sensitive leaked by a media outlet. The reasons for this are myriad, and range from the occasional errant bookstore that puts the title on shelves too early to the unscrupulous reviewer to the intentional-but-made-to-look-accidental leak by the publishers themselves. (This last measure can be surprisingly effective in terms of garnering additional media attention.)
If you're wondering if any of your titles have or will ever be embargoed, cats and kittens, the answer is: unlikely. Unless you're a corporate whistleblower, former Michael Jackson bodyguard, former CIA agent, or J.K. Rowling, publishers probably won't worry enough about the content of your book(s) to keep everything under wraps. Yes, they'll probably be miffed if a book store puts your title on shelves too early, but that generally has more to do with the timing of reviews, co-op, &c than fear that something groundbreaking will accidentally be released too soon.