Strings Content Viewer scans the data of the file / folder and searches it for data that could be text. If it finds data that is ASCII data and at least four characters long, then it displays it to the user.
Note that this is different from the Text Content Viewer, which displays the text for a file that is stored in the keyword search index. The results may be the same or they could be different.
Here's one of the example of "String Content Viewer":