A Spring Boot 'fat' JAR, also known as an executable JAR, is a JAR file that includes your compiled classes along with the dependencies and resources needed to run the application. This means the JAR contains everything required to execute the application without needing an external Servlet container or additional libraries. The process of creating a 'fat' JAR is typically handled by build tools such as the Spring Boot Maven plugin or Spring Boot Gradle plugin. These tools package the application in a way that includes all necessary dependencies in the JAR file.