Used for: converting a list to a normal text field with the list elements separated by a specific character (e.g. comma).
This rule is the opposite of the 'split text to list' rule. In this case, your field already contains a list and you want to convert this to a normal text field (with the elements separated by a certain character). In the example below we will convert a list of sizes to a text field with the sizes separated by "-".
This is what the size field looked like before the rule:
And this is what it looks like after saving the rule: