On the death of Sweyn II his realms were divided amongst his (illegitimate) sons; Viken went to Cnut, Gothenland to Olaf and Denmark to Harald III. Whilst Cnut and Olaf appeared to grab the reigns of power in their respective lands without much ceremony Harald was kept waiting by the Danish nobles. To ensure his election he would have to submit to vows declaring to uphold the law severely limiting his freedom for movement in the coming years.
And so for much of his reign he was dependent on the nobility of Denmark, and in essence did little to oppose them. This gained him his epithet Hen, 'whetstone' or 'soft'. He fought no major wars, taxed little and kept himself busy by reforming Denmark's legal code. Anglian customs such as oath swearing replaced the old trial by combat. Coinage was also improved and official mints established around the country.
Outside of Denmark his half-brother Cnut was constantly threatening invasion, often in league with Olaf, yet papal mediation appeared to calm the situation. Cnut would indeed take the throne but only after Harald died naturally in 1080.
