Daily Rate is applicable only to non-contracted staff (e.g. supply teachers, agency workers, etc) who are paid on an ad hoc/daily basis.
To set a daily rate for a staff member, go onto the staff member's profile in Admin > Personnel > Staff, then click the cog next to their contract.
There is a field labelled 'On Daily Rate' which can be marked to Yes to grey out the fields pertaining to salary information.
Remember to click the green Update button to save the change.