It's not uncommon for Amazon to run discounts on its first-party products, and today is no different. It's currently offering $10 off of the regular price of both the Fire TV Stick and the Fire TV Stick 4K. Regularly priced at $39.99 and $49.99, respectively, that puts them down at $29.99 and $39.99.
The Fire TV Stick is a dongle that plugs into the HDMI port on your TV. Once you connect to Wi-Fi, it will automatically sign into your Amazon account (unless you check the box saying it's a gift when you order it), and you'll be able to watch anything from the library of Amazon Prime Video movies and TV shows. Of course, you can also download a variety of apps such as Netflix, Hulu, and more.
Amazon's Fire TV family of products have become popular for a variety of reasons. One is that they're cheap; after all, $40 is a very low price for the ability to stream 4K content. Another reason is that Fire OS is a variant of Android, which means that you can side-load Android apps to supplement the ones you can get from Amazon's own app store.
If you want to check out the deals on Amazon, you can find the Fire TV Stick here and the Fire TV Stick 4K here. There are also bundles with an Echo Dot, which are $30 off today, since the Echo Dot is currently $20 off of its normal price of $49.99.