what dose this mean When you use a regular expression for the route definition, capture groups are provided in the array using req.params[n]

hi I have been reading on the express docs and when i reached to the req.params i found this line but i did`t understand what dose it mean the line is :

When you use a regular expression for the route definition, capture groups are provided in the array using req.params[n], where n is the nth capture group. This rule is applied to unnamed wild card matches with string routes such as /file/*

can any one help mw with an example to know what dose this mean

the link on express docs are:https://expressjs.com/en/4x/api.html#req.params