A ledger is a record of all transactions that have taken place between different wallets on the Bitcoin blockchain. The Bitcoin ledger is publicly verifiable by anyone and is distributed across the world via nodes and miners.
These nodes ensure that the circulating supply of Bitcoin is not spent twice and the miners ensure that the next block of transactions is processed.
Ledger is also the name of a company that provides you with cold wallets to secure your sats safely.
You’ll often see people use the word “ledger” to describe their hardware wallet, but depending on the context of the conversation, they’ll also refer to Bitcoin’s fully verifiable transaction history.