Record locking during pending approval
Key Takeaway
Record locking stops users from editing a record during approval, even if field-level security or sharing settings would normally let them. In a classic approval process, the default "Administrators ONLY" setting lets a user edit the locked record only if they have "Modify All Records" for that object or "Modify All Data".
When a user submits a record for approval, Salesforce can lock it so it doesn't change while approvers review it. Approvers then approve or reject based only on the record as it was submitted. Classic approval processes lock records by default, and Flow Approval Processes let you choose where the lock starts and ends.
What record locking does
Record locking stops users from editing a record, regardless of field-level security or sharing settings. So a user who could normally edit the record through sharing still can't change it while it's locked. Admins can always edit a record that an approval process has locked.