Why does magnesium oxide have a high melting point?
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Magnesium oxide is an ionic compound ( because it consists of a metal and a non-metal)

In ionic compounds, electrons from the outer shell exchange between the atoms so that they each get a full outer shell. Once these electrons are exchange, the atoms will become ions. One ion will have a positive charge, and the other ion will have a negative charge. The positive and negative ions then bond strongly together in a lattice structure, due to the electrostatic forces of attraction.

These electrostatic forces of attraction are very strong and require a lot of energy to break, as such the compound will have a high melting point.

Now lets look at the ionic bonding between magnesium and oxygen:

In the outer shell of Magnesium, there are 2 electrons.

In the outer shell Oxygen, there are 6 electrons.

Magnesium gives it's two electrons to Oxygen, so that the Oxygen now has a full outer shell of 8 electrons. However, since Oxygen gained two electrons, it will form a negatively charged ion with a charge of -2.

Since Magnesium lost the two electrons, it's second shell (which has 8 eletrons) now becomes the outer shell. So Magnesium also now has a full outer shell. But since it has lost two electrons, it becomes a positively charged ion with a charge of + 2.

The positive Magnesium ion and negative Oxygen ion and now strongly attracted to each other, and form a strong ionic compound (magnesium oxide) with a lattice structure.

The electrostatic forces of attraction between the positive Magnesium and negative Oxygen are so strong that a lot of heat and energy are needed to break them.

Therefore magnesium oxide has a high melting point.