"It is on top of the bed."
Translation:È sopra al letto.
"Sopra" could be over, without touching the referral object. But it can touch the object as well.
"Su" usually touches the referral object.
"Le caramelle sono sul tavolo." = "Le caramelle sono sopra al tavolo." (The candies are on the table)
"Il poster è sopra al tavolo." = "The poster is over (?) the table"
In this case, wouldn't sul be better than sopra? In english "on top of" tends to imply that it is directly on top of the object, touching it. A blanket or pillow is "on top of" a bed, while a light or kid's mobile would just be "above" the bed, because there is space between them and the bed and they do not touch it.