Lord Chaitanya is the Supreme Person, Krishna, appearing as His own worshiper. He is the combined form of Krishna and Krishna's most dear devotee, Srimati Radharani . He came to experience the love She feels for Him, and to show by example how to fully surrender to Krishna.
He exhibited symptoms of love of God to the highest degree, and emphasized chanting the holy names of Krishna as the best means to attain love of God.
References:
- Srimad-Bhagavatam 1.7.13-14, purport—Lord Krishna Chaitanya is K