Some of these items and content include: SwapNotes, demos, videos from the Nintendo Video service, and add-on content from certain games that you've played. Whenever you receive new content via SpotPass, the blue LED light on the upper right corner of your 3DS should light up blue.
If you're not comfortable receiving any SpotPass data, you can opt out of automatic downloads. On your Nintendo 3DS's main nenu, select "System Settings" (the wrench icon). Tap "Internet Settings," and then "SpotPass." From here, you can tap on "Automatic Downloads" to toggle whether or not you'd like your Nintendo 3DS to continue downloading content without your intervention (this is the default setting).
Alternatively, you can also use the SpotPass settings menu to toggle whether or not you want your Nintendo 3DS to send data to other 3DS systems via SpotPass.
Using SpotPass requires a Wi-Fi connection, which is why Nintendo has a record of Wi-Fi hotspots scattered around North America! Check out the HotSpot Locator on Nintendo's website for a hotspot near you.
Learn how SpotPass differs from the Nintendo 3DS's StreetPass feature.