path:   A file name in which we are going to find and replace

regexp: the regular expression to search the string you want

In our case,

() – Grouping of Regular Expression and to create backreferences, which will make it easy to perform backreference while replacing.
^ – Matches the Start of a Line
ServerName – Matches a Literal String
\s – Matches a whitespace
.* – matches one or more of any character/numeral ( not new line)
$ – matches the end of line

replace: A string, with which you want to replace the matched String

In our case,

\1 – backreference indicator and represents the first group we have matched. in our case, it is the result of  (^ServerName\s)

backup:  To tell ansible to backup the file before making any changes.