Lightweight is a simple base theme that's a blank slate with left or right sidebar. The original lightweight which was my internal base theme for D6 had no images whatsoever and very little CSS, thus the name lightweight. The D7 version has a couple of images, but just for buttons and search bar. The buttons, search bar and fieldsets where taken from Bartik, because I fell in love with those elements and they are a good starting point for personalizing them to the theme you're creating.
This theme is for intermediate/advanced themer, that don't need inner divs, starter functions, or that many default regions. It is also for themers looking to have nice markup, there are several override template files for views for example.
If you are just starting out theming better choice might be:
I never really use Lightweight as a parent theme, I use it as a sort of starter kit to start my themes instead of starting them from scratch. The idea is to have starter theme that has a common layout (header - content with sidebars - footer), common template files I always override, and an info file, and then modify the files and directory to make it your own.
The simple instructions to start are: