A rondel was a circular piece of metal used for protection as part of armor. The Armorer forged a rondel for the foundling Grogu from beskar scraps donated by other Mandalorians for foundlings. She personally added it to Grogu's armor saying it would protect him as he grew into his new station as a mandalorian.
The rondel had the symbol of Clan Mudhorn. During the Conflict on Mandalore, Grogu's rondel did indeed protect him during his fight with the Imperial Praetorian Guards when a heavy metal bar fell on top of Grogu.