Because mother Saci was feeling separation from Sri Caitanya Mahaprabhu, she thought she was dreaming that her son had come to her. Sri Caitanya Mahaprabhu, however, wanted to inform her that actually it was not a dream. He actually came there and ate whatever His mother offered Him. Such are the dealings of advanced devotees with the Supreme personality of Godhead. As stated in the Brahma-samhita:

"I worship the primeval Lord, Govinda, who is always seen by the devotee whose eyes are anointed with the pulp of love. He is seen in His eternal form of Syamasundara, situated within the heart of the devotee." (Brahma-samhita 5.38) pure devotees realize dealings with the Lord on the transcendental plane, but because the devotees are still in the material world, they think that these are dreams. The Lord, however, talks with the advanced devotee, and the advanced devotee also sees Him. It is all factual; it is not a dream.