Trolls were extremely feared in past times in Scandinavia. People would carve large crosses on their front doors to keep them away.
According to tales, they loved to eat humans and would steal your children if you weren’t careful. Sometimes they were almost identical to humans in appearance, sometimes they would use magic to look like rocks and trees to blend in with the environment, and sometimes they were huge and giant like (pic related). In some writings, they’re the same things as giants (Jötunn).
People used to believe that huge rocks randomly scattered around as a result of the iceage) were placed there as a result of angered giants and/or trolls hurling them against christian churches.
People also believed that it was possible for trolls to summon themselves to wherever you were if you talked about them, hence the saying “speak of the trolls”.