Base Price: Original price for your product.
Sales Price: Promotional price for your product, if you set a sales price ( lower than base price & greater than 0 ), it will override base price during checkout process and you can set an expiry date for the sales price, after the expiry date, it will apply base price as product price.
Check this option if your product has multiple licenses and different pricing options. From listed license options, you can enable different license levels and set price for them. You can manage licenses from Admin Menu >> Downloads >> Settings >> Premium Packages tab:
You also can set file based pricing, you simply need to put prices on the right of individual file item:
At frontend it will look like this:
This option is very useful, when your site has paid membership options and you want to allow free download or a certain percent of discount for some membership levels ( user roles ). Suppose, you put 100% discount for "Administrator" role. Now, all administrators will be able to download the product for free, but other users need to pay the price to get access to download.
You can create one for more coupon codes with different discounts.