amiibo are figurines with an embedded NFC chip created by Nintendo that when scanned with a Wii U, New Nintendo 3DS, or Nintendo Switch, can unlock certain content. Older 3DS models use the NFC Reader/Writer which was released in 2015.
Functionality for EarthBound series amiibo
Here is what happens when you use the Ness and Lucas amiibos with a fewgames.
Super Smash Bros. for Nintendo 3DS & Wii U
In Super Smash Bros. for Nintendo 3DS and Wii U, with amiibo, players can create and train their own CPU fighter. Players can customize their amiibos with any special moves they unlocked or by changing their stats by feeding them equipment.
Super Smash Bros. Ultimate
All amiibos that were compatible with Super Smash Bros. for Nintendo 3DS and Wii U will be compatible with Super Smash Bros Ultimate. Fighter data from the previous games will carry over when used in Super Smash Bros. Ultimate.
Scanning any amiibo besides any Yarn Yoshi amiibo will make Yoshi look like whatever character was scanned. Using a Ness amiibo unlocks a Ness Yoshi. However, using a Lucas amiibo unlocks a Yarn Yoshi with a generic design of the amiibo logo.
In Miitopia, scanning many amiibos will unlock costumes for playable mii characters. Scanning the Ness amiibo will unlock a Ness hat and outfit.
Other amiibo compatible games
Many games that are compatible with amiibos allow you to scan any amiibo. The user will normally only be able to scan an amiibo once per day and get something generic like some form of currency. Some of these games include Hyrule Warriors, Mario Party 10, Captain Toad: Treasure Tracker, and Pokkén Tournament.