Activators are tools used to activate software, bypassing the standard activation process that requires a valid product key. They work by emulating a Key Management Service (KMS) or by directly manipulating the software's licensing status. While they can be used for legitimate purposes, such as activating a trial version of a product or a version of Windows that was previously activated but is now requesting reactivation due to hardware changes, their use without a valid license can be considered piracy.