Those little metal dots are the sensors that allow Core to give you biofeedback. Using surgical grade stainless steel sensors and the same technology that your hospital would use to check on your heart, Core is able to measure and track how meditation is affecting you.
As you meditate, do your best to keep your thumbs over the sensors. It should feel natural to have your thumbs gently rest on them. In the case you set your Core down or take your thumbs off the sensors, this will result in gaps in your data.
Don't worry about having your thumbs cover all the dots, just make sure you're touching a few on each side so that Core can get an accurate assessment.