The LOCK ALL command locks all fields on the current slide and changes the icon on the LOCK ALL button to show a closed padlock, indicating that the fields are now locked. 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. Only when fields are unlocked can their text be edited.
The open padlock icon of the command button indicates that (some or all) fields are currently unlocked. Clicking on the button when that icon shows will lock all fields and cause the icon to change to a locked padlock.