Publishers are Electron Forge's way of taking the artifacts generated by the make command and sending them to a service somewhere for you to distribute or use as updates. This could be your update server or an S3 bucket.
Each publisher has to be configured in the publishers section of your Forge configuration with which platforms to run for and the publisher specific config. For example:
Please note that all publishers default to publishing all platforms, so you only need to specify the platforms key if you don't want that default.