Amazon Simple Queue Service
Amazon Simple Queue Service, or SQS, is a hosted message queue for sending messages between distributed components of a web application. It is a service under the AWS umbrella with a generous free tier.
SQS is probably overkill and/or not useful for most self-hosted IndieWeb applications. At first glance, it appears helpful in running asynchronous jobs on a shared host. But unfortunately you must periodically poll the endpoint for new jobs, rather than having jobs sent to you through eg. a webhook, so you would still need to have a continuously running process to monitor SQS and that's what can be hard on a classic shared host.