As I explain in my books, the yellow line indicates the need for you to render your timeline, either because you've added effects or transitions to your timeline or you are using media files that are being interpreted to your project specs.
To render, press the Enter key on your keyboard or click the Render button on the timeline. The yellow line will be replaced by a green line.
Welcome to the forum.
As Steve points out, the "yellow line" has replaced the "red line," as of PrE 11.
For more info on Rendering, and what it does, see this article: http://forums.adobe.com/message/5155114#5155114. It has been updated to account for the change in PrE 11.
Also note: Premiere Pro still have a red line, yellow line, and green line, but in that program, the yellow line has a slightly different meaning. Should you run across references to PrPro's yellow lines, you can just skip over those, if you have PrE 11.