An amiibo’s Learn Button is a featured introduced in Super Smash Bros. Ultimate. Hence its name, it determines whether or not the amiibo learns from opponents during matches.
Switching an amiibo’s Learn button to ON is the only way to change its Style without feeding it Spirits. Learning on is recommended when leveling up an amiibo or teaching it important strategies after it reaches Level 50. It is worth noting that amiibo with Learning on are actively trying to copy their opponents’ play style, not counter it. For example, if one attempts to train a Mario amiibo while playing as Ganondorf, the amiibo will adopt the character’s playstyle even if it is fundamentally incompatible between both movesets.
Switching an amiibo’s Learn button to OFF is somewhat misleading, as it still learns new information. With Learning off, its Style cannot change, but its move priority can. Most Level 50 amiibo submitted to tournaments have their Learning turned off so that they are not corrupted by poor-playing opponents. Learning off is often used to teach an amiibo matchups against certain characters. Switching an amiibo’s Learn button to off and raising it to Level 50 is not recommended.