Similar to the way the Find and Replace function in a text editor works,
Find and Replace mode lets you perform a batch rename by specifying a
text string to search for and another to replace it with.
This example has the Find string (note the Old name field has been replaced with Find) set to img, and the Replace string is set to IMG. The preview list indicates the outcome of the batch rename - as you can see, only two of the displayed files will actually change as a result of the rename (they are the two displayed in black - the rest, in grey, won't change).
Ordinarily the search is confined to the filename stem, excluding the file extension. If the Ignore extension option is turned off, however, the Find and Replace operation will be performed over the whole filename including the extension.