Mark a field read-only to ensure that it is accessible but cannot be edited or inadvertently tampered. You can set the field read-only either while adding a new field (custom fields) or editing an existing field (default and custom fields).
Here's how you can do this:
Go to Admin settings > Leads, Contacts & Accounts > Contacts/Accounts/Custom Module > Add Fields.
For deals, navigate to Admin settings > Deals & Pipelines > Deals > Add Fields
Click the dropdown icon on a field and Enable the Read-only check box for a field.
Alternatively in the Add Field/ Edit Field overlay, you can enable Mark this field as a read-only checkbox.
Click Save
Note: Only the fields that are not required (mandated), can be made read-only. i.e: For all required fields, the read-only option is disabled.