The UNLOCK ALL command unlocks all fields of the current slide so their text may be edited, and changes the icon of the UNLOCK ALL button to an open padlock to indicate the current state of the fields.
Only when fields are unlocked can their text be edited. Only when fields are locked will clicking on them be understood as a signal for setting them into action. Otherwise it is understood as part of the process of editing their text.
The closed padlock icon of the command button indicates that (some or all) fields are currently locked. Clicking on the button when that icon shows will unlock all fields and cause the icon to change to an unlocked padlock.