Theme and subtheme basics
You haven't really asked a question. But i'll assume that you want to know what a base and sub-theme is in drupal.
TLDR version
A drupal subtheme inherits the template files (*.tpl.php) and assets (javascript, css, images) of the base theme. If you need to modify or change the template files.
More on subthemes
I think your best bet is to google it. I googled, &quot;drupal subtheme&quot; and found you  drupal documentation regarding subthemes:

Sub-themes are just like any other theme, with one difference: They
inherit the parent theme's resources. There are no limits on the
chaining capabilities connecting sub-themes to their parents. A
sub-theme can be a child of another sub-theme, and it can be branched
and organized however you see fit. This is what gives sub-themes great
potential.

Source: Creating a sub-theme (drupal.org)
In addition to the node page, if you're more inclined to watch a video this is probably a good bet: Drupal Subthemes (youtube.com)
Why should you avoid modifying a base theme

Flexibility: Base themes maintain flexibility. If there is an update, your child theme inherit these changes
Losing your changes: Modifying your base theme means that when an update to the theme comes along your modifications could potentially be lost
Maintain simpler code base: The base theme allows you to abstract a lot of functionality and reduce the code base. This means less maintenance in the long run.

Best Practices with subthemes:

Copy files from the base theme or use the drush command (when available)
Utilize subtheme theme hooks to override your theme