Transactions represent significant interactions or events (e.g. user onboarding, loan application journey, etc). While an Account in Persona represents a user or entity, a Transaction captures some event associated with that user/entity.
The Transaction Object is itself generic and unstructured, but you can define the object type, field schema, and status to model any event of relevance to your own organization.
Transactions are integrated via Workflows and tie automations across Persona objects and constructs. Transactions and Workflows connect all parts of the platform together and back to your own systems over time.