This was a part of the high priest's dress, which he wore when performing his office in the temple service. On this breastplate were engraved the names of the twelve tribes of Israel, and it was called, "the breastplate of judgment." (Exodus 28:15) The design of it seems to have been to typify the Lord Jesus Christ, the great and almighty High Priest of his redeemed, who going in before JEHOVAH, bears the names, and persons, and concerns of all his people. Hence, the church so vehemently desired the Lord that she might be set "as a seal upon his heart, and a seal upon his arm." The former the tenderest, and the latter the strongest part in Jesus's affection. (Song of Song of Solomon 8:6) And hence, in allusion to the same, the apostle exhorts the church to put on "the breastplate of faith and love;" meaning, a steadfast looking unto Christ in the exercise of those graces, by relying wholly on him for mercy and salvation. (1 Thessalonians 5:8)

Bibliography Information
Hawker, Robert D.D. Entry for 'Breastplate'. Hawker's Poor Man's Concordance and Dictionary. https://www.studylight.org/dictionaries/eng/pmd/b/breastplate.html. London. 1828.

Bible Dictionaries Breastplate

Hawker's Poor Man's Concordance And Dictionary

Bible Dictionaries
Breastplate