Jamieson, Fausset and Brown on 1 Kings 11:3

3. he had seven hundred wives, princesses--They were, probably, according to an existing custom, the daughters of tributary chiefs, given as hostages for good conduct of their fathers.

concubines--were legitimate, but lower or secondary wives. These the chief or first wife regards without the smallest jealousy or regret, as they look up to her with feelings of respectful submission. Solomon's wives became numerous, not all at once, but gradually. Even at an early period his taste for Oriental show seems to have led to the establishment of a considerable harem (So 6:8).