There are two things you can lock: Transactions and Accounting spreads.
General difference:
Transaction locking
Transaction locking doesn’t allow you change anything (name, dates, amounts, revenue recognition, etc)
The lock date looks at the start date of the transaction
Accounting Spread locking
1) You are able to change items in the transaction
The only thing that is locked is the Accounting Revenue. The accounting Revenue will remain the same even if something about the transaction is changed.
The lock date will lock anything in Accounting Revenue before that date
Accounting Spread locking - more details:
These are additional (more granular) cases where the accounting spread will be locked: