The word medelsvensson can be translated as 'typical Swede', and it's used to talk about the average Swedish inhabitant. It comes up a lot when talking about data and stats.
For example, you might say something like medelsvensson dricker många koppar kaffe varje dag (the typical Swede drinks a lot of cups of coffee every day), or a researcher might ask hur stor är skillnaden mellan vad en medelsvensson och de rikaste tjänar? (How big is the difference between what the average person and the richest earn?).
You can also use it to mean 'an ordinary person' or 'the man on the street' in other contexts.
Medel means 'average' (or 'mean' to be specific), and the term comes from the fact that Svensson is such a common name in Sweden.
As of December 2018, there were 93,225 people in Sweden with Svensson as their surname. That's not even counting spelling variations such as Svenson and Swenson, or the 64 men and five women who had Svensson as a first name.
In other words, roughly one in every 100 Swedes has the surname Svensson, but surprisingly that still doesn't make it the country's most common last name. In fact, it only comes ninth on the list with Andersson the most popular: 231,727 people in Sweden have the name. The next most common surnames in 2018 were Johansson, Karlsson, Nilsson, Eriksson, Larsson, Olsson, and Persson.
But the reason we talk about medelsvensson when we talk about the average Swede is that the term was first coined back in 1930. At that point, Svensson was in fact the most common surname, so it made total sense.
You might also hear people use the term svenssonliv (a 'Svensson life'), which basically means 'an average life' in the Swedish context.
You could draw a lot of conclusions from the popularity of these terms, and many people would point to Sweden's reputation as a culture which prizes conformity and egalitarianism. As the country develops, partly due to globalization and partly due to immigration, it remains to be seen how the concept of the medelsvensson will change.
Medelsvensson heter faktiskt Andersson
The typical Swede actually has the surname Andersson
Familjen Medelsvensson har förändrats under årens lopp
The typical Swedish family has changed over the years