This option determines whether the last modification date of all objects are shown in the object list. Together with the objects names you can configure LEX to show any object attributes to be shown directly in the list as additional columns:
There are five important basic information for each object, which can be chosen directly to be shown as a list column or not. The Date Modified is one of them: This is the standard attribute modifyTimeStamp which is defined in the LDAP v3 RFC 2251 and which is supported by almost every directory server.
You could configure the Date Modified option also by opening the context menu of the object list column header - just right-click on the column header: