Gaiden chapter

From Fire Emblem Wiki

A gaiden chapter (A.K.A side-quest) is an optional chapter that is played when the player has met certain conditions. Gaiden chapters were introduced in Fire Emblem: Thracia 776. They often feature fog of war.

They are not part of the main story, but unlocking Gaiden chapters may impact the ending. For example, in Fire Emblem: Fūin no Tsurugi, unlocking a Gaiden chapter is required to access the "true" ending of the game (since the player needs to obtain all legendary weapons and have them intact while facing King Zephiel in order to unlock a few extra chapters that will lead to the the real final boss). However, in Fire Emblem: Shadow Dragon, Gaiden chapters are chapters that players should try to avoid, as most of the chapters require you get a large number of your units killed in order to access the chapter.