3 aspects of the same being, perfectly united.
If the concept seems baffling, think of how we consider our bodies to be individual bodies, and yet they are each composed of a trillion cells.
Or think of how a human being is considered to be mind and body, yet 1 person.
The biblical doctrine of the trinity is not one that is named such in the bible, but study of the bible shows that God acts and reveals Himself throughout scripture through the embodiment of His word, spirit, and mind (the father).
As humans made in the image of God (though we are corrupt), we are like Him in some regards.