To put it very simply, everyone makes mistakes. Even with the best employees and the best risk management practices in place, mistakes will be made. No one is perfect.
Errors and Omissions (E&O) is the insurance that covers your company, or you individually, in the event that a client holds you responsible for a service you provided, or failed to provide, that did not have the expected or promised results. For doctors, dentists, chiropractors, etc., it is often called malpractice insurance. For lawyers, accountants, architects or engineers, it may be called professional liability. Whatever you call it, it covers you for errors (or omissions) that you have made or that the client perceives you have made.
Most E&O policies cover judgments, settlements and defense costs. Even if the allegations are found to be groundless, thousands of dollars may be needed to defend the lawsuit. They can bankrupt a smaller company or individual and have a lasting effect on the bottom line of larger companies.