The 'Sample Downloads' included with your Plan act as a way for artists to test images and create concept mockups without spending any Credits.
If enabled for a Team Member, they will have the ability to download any image at 1600px resolution for 'free' providing your account hasn't hit it's cap for that day. The number of these downloads you receive per day is based on the number of Site Licenses your account has. Each Site license comes with 25 free sample images per day.
Sample downloads don’t rollover and they don't affect the Account's Credits in any way.
You can also set how many free images each team can use, I.e. you could divide them up equally or, say, create a 'Concept Artist' team with zero allocated credits but allocate them all the free downloads on the account - this would then allow the concept artists free reign over usage of images on the site without affecting the credits used.