A sequence is an array of zero or more elements of a given type. For example, the system.schema.yaml defines a configuration object system.mail that contains:

system.mail: 
  type: config_object 
  label: 'Mail system' 
  mapping: 
    interface: 
      type: sequence 
      label: 'Interfaces' 
      sequence: 
        type: string 
        label: 'Interface' 

The export would contain an attribute called interface containing an array of elements that would be treated as strings. Any type can be used in the sequence, including scalar and complex types.