A ledger is a record of a financial transactions - an invoice, a credit memo, a payment, etc. When present, ledger entries are listed together on the main ledgers-node, with each ledger type having a dedicated subnode:

You can click on any of the ledger entries to open an overview screen where you can see information such as:
- The current ledger state
- Order lines - products, discounts, fixed lines, etc.
- Shipping & Billing details
- Order totals
- Payment details
- Etc.
Ledger entries are typically imported to DynamicWeb from an external system and then displayed to customers in frontend in a Customer Experience Center or similar.