This is when using one of the variable text options, like serial numbering or CSV/Merge. All text shapes have an ‘offset’ value which is added to the current serial index, so it can be used to offset a number, or look up a different entry in a CSV file.
For a quick example, if I create a single text object and set it to ddd (3 decimal digits), and set the type to ‘Serial Number’, when copied with the Grid tool, the index is incremented for each copy automatically.
In the edit window it looks like this:
… but when previewed, you see the actual indices being used:
