Social democracy is an ideology for a competitive economy with money while also helping people whose jobs don't pay a lot. This makes it easier for people to support themselves in society by having more protection if they get fired, as well as government ownership or regulation of various industries that make companies play by their rules, and an emphasis on equal rights for everyone. This way of governing is appreciated by people living in countries where being in the cold has a more down-to-earth meaning, like the northern nordic countries of Sweden, Denmark, Finland, Norway, and Iceland. The Social democratic movement has seen some interest in others countries since their systems do not work this way.
Before any country used a social democratic system, there were two ideologies that had the idea of making everyone equal in society; socialism, the idea of the workers owning everything, and communism, the idea of the workers controlling everything. A small group called democratic socialists hated how too much capitalism led to people becoming poor, and how too much communism led to almost no competition between anyone. So they made a compromise, or balance, that makes some people rich but leaves no one poor.