Best before: Still good, although the quality may not be as good/effective. As protein (powder?) is quite processed, you should be fine.
Use by: Use the product by this date or risk food poisoning. Applies heavily to dairy or very perishable foods.
Also, these dates are usually printed a couple of days before the product is really 'off'. Why? food manufactures don't want to have to go through law suits or have their reputation tarnished because people start getting sick from their product.
Bottom line, you'll be fine.