Complex sentence: The song's title is a reference to Tenerife in the Canary Islands of Spain, where the seas are blue that Sheeran likened to his girlfriend's eyes that are "seriously blue, like electric blue".
Split into simpler sentences: The song's title is a reference to Tenerife in the Canary Islands of Spain. According to Sheeran, the hue of his girlfriend's eyes are as blue as the seas in Tenerife, "seriously blue, like electric blue".