Ansgar (c. AD 800-865) was a missionary for the archiepiscopal see in Hamburg-Bremen, which had the Nordic Countries as its mission area.
There were probably quite a number of German/Frankish missionaries in the Nordic Countries, but Ansgar is one of the few we know of, in particular because a biography was written of him. There was a wish to have him declared a saint after his death, so it was necessary to ensure that his life history was written down, together with all his deeds and achievements. Consequently, we must be rather cautious in believing everything that was written down; perhaps there was some exaggeration in order to make it easier to have him beatified.
Ansgar's missionary work was among the Saxons, Danes and Swedes. In addition to the fact that the monks, of course, wished to convert as many people as possible to Christianity, conversion of the Nordic peoples was also important for the Frankish Emperor. Perhaps these warlike neighbours would give fewer problems if they became Christians.
Ansgar's first missionary expedition was in AD 826, during which he spread the Christian message among the Saxons and possibly also the Danes of Southern Jutland. But shortly afterwards, in c. AD 830, he travelled to Sweden, where he established himself in particular in Birka. According to Angar's Life History, he converted many people here and obtained permission to build a church. This is the earliest church in the Nordic Countries.
Back in Hamburg, a couple of years later he was appointed as Archbishop of Hamburg diocese, which also had the Nordic Countries within its area of responsibility. Subsequently, he functioned in particular as a diplomat rather than an actual missionary, acting between the Danish King and the Frankish King/Emperor. He was given permission to build churches in both Haithabu and Ribe.
On Ansgar's death, in his sick bed in AD 865, Christianity had not become particularly well established in the Nordic Countries. It was first with the arrival of Harald Bluetooth that Christianity really took hold.
So when Ansgar is referred to as the "Nordic Apostle" this is probably somewhat of an exaggeration.