The term concatenate means to put pieces together to make a whole. In the case of InfoPath, the CONCAT function allows you to put any combination of the following elements of your InfoPath form together into one element:
concat(fieldname, "text", function)
Use the CONCAT function to give a form a name when submitting to a Sharepoint list.
concat(FullName, "-", today) would result in the form being named DebbieSilbert-11/10/06.xml.