When this mode is enabled (i.e. the button state is pressed), any zoom action on a photo will affect all other open photos in the same way. Zoom one photo in, and the rest will zoom the same amount. If photos are zoomed in a different amount before enabling the lock, all photos will zoom to the same level after the lock is engaged. If there are any selected items, the zoom will keep the selected items centered.
This tool is available when a photo window is active.
See Preferences - User Interface Image pane) to set whether the Zoom lock tool is enabled by default.