The ‘Loch Ness Monster’ is not that kind of ‘Lock’. Regardless, you want to lock monsters from ruining your system with bad data entry.

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Lock Fields

In personalization, Lock allows users to view fields, but not change their values. To lock a field, right click, select personalize, and then check the lock button.

As with all personalization features, these locked fields can be distributed to other users as published saved views.

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Can’t This Be Done Through Security?

Yes. Without personalization, controlling who can see or edit what fields is managed through security. If you click on security diagnostics for the Vendor form, you can see there are two security privileges: one to view vendors and one to maintain (edit) vendor data.

To lock down a subset of fields through security, a system administrator would need to create a new security privilege (or modified copy of the maintain vendors privilege) and assign it to the desired duties or roles.

An advantage of locking fields through personalization is that it is easier to see which fields are being locked through the UI. Also, users can have multiple views – It’s possible have a default view where the data is not editable, but allow the user to toggle to another view where the data can be edited. This would be a more difficult proposition to manage through security. Pro tip – if you care very much about tracking edits to a particular field, consider enabling database logging on that field.

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Lock Many Fields

Personalizations can be applied to individual fields, as well as Field Groups or FastTabs. If you want to lock lots of fields on a form, the easiest way to do this is by opening the personalization toolbar, clicking Lock and then clicking on the desired Field Group or FastTab.

Clicking on the Payment group controls all fields in the Payment group

Clicking on the Payment FastTab controls all fields in the Payment FastTab

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That’s pretty much the whole concept – Lock, Stock, and Barrel.

Additional Reading
Personalize the user experience
D365 security topics
Database logging