Here is the formal definition for the asterisk:
* Match zero or more of the previous items
If the only special character you are using is the star *, then the previous item is defined as the previous character. For example, let's say that your company uses five-digit part numbers in the format PN00000 and you want to know how many people are searching for part number 34. Technically, the part number is PN00034. So you might use the keyword filter box at the bottom of your keyword report with this expression: PN0*34. That will display all the searches for PN034 and PN0034 and PN00034 and PN00000034 and for that matter, PN34, since using the star means that the previous item doesn't need to be in the search - zero or more of the previous items, it says.