points out that there are no conditions. The closed lock 
indicates the opposite.
When the icon is on grey, the conditions cannot be edited (you have to select the corresponding line of the table). When the icon has a color, a click over it allows the edition of the condition throught the conditions dialog.
The open lock points out that there are no conditions. The closed lock indicates the opposite.
When there are no conditions () those are always either active or valid.