Hello,
EXTRARES are usually the gifs, images and javascripts that you have running on the page that you are trying to capture. These get recorded as part of your script based on your recording option. You can choose not to record these components also. The EXTRARES are loaded parallely as your page gets loaded in order to get the complete page. The EXTRARES has a significant impact on your response time of the page and you can check this by commenting out the EXTRARES part.It is normally advisable to keep the EXTRARES and work as it simulates the actual page loading and hence gives you the right performance numbers.
I want to add that this reponse was given by Prashanth Thimmavajjala from another similar topic, however I wanted to share it with you.
Hope can helps you.