Karna’s protection (kavacha) was his golden armor and earrings. He was born with both of them.
Indra (Arjuna’s father) knew that he could not be killed and thought about how he could take it away from him. Surya (Karna’s father) appeared to Karna in a dream, revealed Indra’s plan to him and advised him not to give up either.
But Karna was ready for it. Surya advised him to get his best weapon, Vasavi Shakti, from Indra as a replacement. When Indra appeared in the form of a wise man and asked Karna for kavacha and earrings, he tore them both from his body and ears and handed them over to the supplicant. Then he asked for Vasavi Shakti, which the sage gave him, with the remark that he could only use it once.