Ranked armor is visual representation of your rank in League of Legends, you can check how it looks and what parts are added after each split below.
Starting in 2019 all League of Legends seasons will be divided into 3 splits, each giving you unique rewards for playing ranked. You can check your current progress and when split is going to end by going to Profile->Ranked in LoL client.
Ranked armor and other rewards
Each split you will get a different part of ranked armor. Ranked armor and emote dynamically updates so if you got your split one reward when you were gold it will update to higher rank once you reach it. Your ranked armor is visible everywhere in the client and will be shown as previous rank in the next season.