A Marketo custom object allows for the creation of a one-to-many relationship between your Marketo Leads and the custom object records. When the Marketo custom object is successfully provisioned in your account, you can:
Create/Read/Update/Delete records in the custom object via Marketo SOAP API
Use smart list trigger when new records are added to the custom object
Use the custom object data as a filter in smart lists (segmentation)
Use the custom object data in emails using [Marketo email scripting]
Reference:
Getting Started