serverless/docs/guides/variables/self.md

Read this on the main serverless docs site

Self-Reference Properties in serverless.yml

To self-reference properties in serverless.yml, use the ${self:someProperty} syntax in your serverless.yml. someProperty can contain the empty string for a top-level self-reference or a dotted attribute reference to any depth of attribute, so you can go as shallow or deep in the object tree as you want.

service: new-service
provider: aws
custom:
  globalSchedule: rate(10 minutes)
  # the following example purposely demonstrates the ability for a variable to cross reference another one
  serviceName: ${self:service}
  exportName: ${self:custom.serviceName}-export
  # or simply
  # exportName: ${self:service}-export

functions:
  hello:
    handler: handler.hello
    events:
      - schedule: ${self:custom.globalSchedule}
  world:
    handler: handler.world
    events:
      - schedule: ${self:custom.globalSchedule}
resources:
  Outputs:
    NewServiceExport:
      Value: 'A Value To Export'
      Export:
        Name: ${self:custom.exportName}

In the above example you're setting a global schedule for all functions by referencing the globalSchedule property in the same serverless.yml file. This way, you can easily change the schedule for all functions whenever you like.