Under unix-like systems, all directories contain two entries,
.., which stand for the directory itself and its parent respectively. These entries are not interesting most of the time, so
ls hides them, and shell wildcards like
* don't include them. More generally,
ls and wildcards hide all files whose name begins with a
.; this is a simple way to exclude
.. and allow users to hide other files from listings. Other than being excluded from listings, there's nothing special about these files.
Unix stores per-user configuration files in the user's home directory. If all configuration files appeared in file listings, the home directory would be cluttered with files that users don't care about every day. So configuration files always begin with a
.: typically, the configuration file for the application Foo is called something like
.foorc. For this reason, user configuration files are often known as dot files.