Packer is a tool for creating machine and container images for multiple platforms from a single source configuration. It allows users to automate the creation of machine images by defining infrastructure in configuration files and running builds that put everything needed to reproduce the machine into an image. Packer templates define builders, provisioners, variables and other configuration to automate the creation of images. This simplifies deployment by allowing images to be built once and then easily deployed to different environments like development, testing and production.