Two different things make an account locked, both shown as a disabled row with no Edit pencil and, if opened, the banner "This account is locked. System accounts can't be edited — they're managed automatically." with every field read-only and no Save button:

• Whole locked groups — every account in Sales Taxes, Uncategorized Income, Uncategorized Expense, Gain On Foreign Exchange and Loss On Foreign Exchange is locked, and you can't add new accounts to those groups either (no Add a new … account link).
• Specific starter accounts in otherwise-editable groups — a handful of the accounts Sprwt creates for you are individually locked even though other accounts in the same group can be edited: Payroll Clearing (Money in Transit), Accounts Receivable (Expected Payments from Customers), Accounts Payable (Expected Payments to Vendors), Payroll Liabilities (Due For Payroll), and Payroll Employer Taxes / Payroll Gross Pay (Payroll Expense). Any other account you add yourself to those same groups can be edited normally.

Attempting to save a locked account returns Account is not editable.