the webroot is the location where your html files are on your machine.
so if the webroot was C:/webroot/ with a file called bob.html in that directory you could access the file by going to
http://localhost/bob.html
you can set the web root in the httpd.conf file which is a text file containing apache configuration. After adjusting httpd.conf you must restart apache for the changes to be made.
the forward slashes in the path "C:/webroot/" should be back slashes but my mac doesn't seem to want to give me a backslash