That is indeed the adhesive, holding down the screen in addition to the screws and securing it to the frame. It is nothing to be worried about; in fact, after a repair the old adhesive is usually discarded and replaced with new ones to ensure the screen stays adhered. In some phones it is also used to make the phone waterproof by sealing up the crack between the screen and the frame. However, the iPhone 6s is not waterproof, so you can even choose not to install it. It is best to replace it though, as the adhesive helps prevent dust from entering the device.