This mode searches an image for a given string. This is most useful when searching for deleted content. To decrease the time required for a search, a "strings" file can serve as an index. This file will contain only the ASCII strings in the image.
Autopsy will also prompt you to create a file of unallocated data if one does not exist. This obviously is useful for recovering deleted data. If a string is found in this file, Autopsy will also report the location in the original image.
If you have not generated a strings file or unallocated data file yet, that option will exist.
The Load Unallocated Image or Load Allocated Image button exists to switch between the two file types if they have both been generated.
Autopsy also has the ability to perform pre-configured searches. They are shown in the "Predefined Searches" section.
If the search was done on an unallocated data file, then an option will exist next to each address to also view the original. Doing so could reveal the inode that allocated it.