The Standard Rename mode lets you batch rename files using a simple wildcard system. The * character (asterisk) is used to specify one or more parts of the existing filename that are to be retained in the new name.
In the above example, you can see that the original filenames had a mix of Img_ and IMG_ prefixes, and the file extensions were also a mix of upper and lower-case.
The Old name pattern of img_*.jpg tells Opus to match any filename beginning with img_ and ending with .jpg (case does not matter as the Case sensitive option was not enabled). The * in the pattern is used to mark the portion of the filename that will be carried through to the new name.
You can use as many instances of the * character in the patterns as you like, as long as there are at least as many in the Old name field as there are in the new. For example, if you wanted to preserve the case of the file extension in the above example, you could specify an Old name pattern of img_*.* and IMG*.* for the New name pattern.
You can't use this system if you want to change the order of preserved strings (as one asterisk looks very much like another) - to do that, you need to use Regular Expressions mode, which lets you refer to preserved strings by number.