If you are not sure which directory in a WordPress theme is the main directory for it, locate the directory with a style.css file in it.
Knowing what a themes root or main directory is can be helpful in knowing which directory to upload to your themes directory, where to place new files for the them to override default functionality, etc.
Theme root for themes already installed
For themes that are already installed in WordPress, the root directory is typically a directory with the name of the theme located in "wp-content/themes" on your server.
Theme root for extracted themes
Most people who write themes have the extracted folder as the root directory for the them. This should always be the case for themes downloaded from WordPress.
However, if you want to double-check, open the main directory for the extracted theme and see if there are files in it. If there are only directories (folders) in it, then look through them until you find a folder with the file "style.css" in it. That is your themes root directory…and the directory containing it is the one you want to upload to use the theme.
For example:
If you had downloaded a theme named "whatever" and extracted it, you probably created a "whatever" directory when you did so.
If the "whatever" folder has a style.css file in it, then you know to upload the "whatever" folder to "wp-content/themes" on your server.
If, however, the "whatever" folder only had another folder in it named "nevermind", you would open that folder. If it contained a "style.css" file, then you would upload the "nevermind" folder to "wp-content/themes" to get the theme to show up for selection in WordPress.
