Banners are a cosmetic feature for StarCraft II. They were first introduced with the BlizzCon 2018 War Chest.
Banners are displayed when players load a game in Versus or Co-op, with two unlocked by default for each race (including random). Each War Chest comes with its own race-specific banner. If players purchase all three, they will receive an exclusive "random" banner (used when the "random" option is chosen for multiplayer games).