It's hard to say more with the few details that I have.
Service Broker is something completely different. It's an asynchronous messaging service and can be used for a number of things. But there are situations where Service Broker can be an alternative to Agent. Say that when a row is updated in a table, you want some data to be recalculated, but you don't want that to happen directly inside the transaction, as it was take to much time. One solution is to have an Agent job that runs periodically and scans for changes. Another is to post a message on a Service Broker queue, and this fires an activation procedure which runs separately and performs the recalculation.
Service Broker is not a scheduler, but you can set up conversation timers, so that an activation procedures fires periodically to perform an action. Since everything is stored in the database is internal tables, you will not miss actions because something was down at the time.
If you want to learn about Service Broker, this book is a good start:
https://www.amazon.com/Rational-Server-Service-Broker-Guides/dp/1932577270/ref=sr_1_1?dchild=1&keywords=Roger+Wolter+Service+Broker&qid=1626856717&s=books&sr=1-1. Roger Wolter was the Program Manager for Service Broker when it was originally developed, so he knows what he is talking about. And it is a very neat book - it's only just over 200 pages.