The disciples were very afraid, and this reaction makes sense. People cannot walk on water, so they thought they were seeing a ghost.
As in many times when God appears (divine manifestations, when God shows himself), the Lord spoke words of comfort: "Take courage! It is I. Do not be afraid."
It was not a ghost but Jesus (their friend, Savior, and Lord).
It is I : Jesus walking on the sea and calming the storm were miracles (acts of God’s power). Because of this, Mark’s first audience would have understood the words I am as more than a simple statement. They would have heard a connection to how God describes himself in the Old Testament (Exodus 3:14; Deuteronomy 32:39; Isaiah 41:4; 43:10–13). This suggests that Jesus shares in God’s identity (he is divine).