Smörgås means 'sandwich', but is most familiar to non-Swedes as part of the compound noun smörgåsbord. In Swedish, this refers to a buffet of open sandwiches, but in English it's usually used metaphorically to talk about a wide variety of something.
When we say 'open sandwiches', this means that a Swedish smörgås wouldn't really be defined as a 'sandwich' at all in English, where the most defining characteristic of a sandwich is that the filling is placed between two slices of bread. The verb 'to sandwich' (meaning 'to put in the middle of two things') illustrates this.
But in Swedish, a smörgås is usually one slice of bread, with butter and a filling -- perhaps meat, vegetables, or cheese -- placed on top. You can also use the word dubbelsmörgås (literally 'double sandwich') to describe a sandwich featuring two slices of bread.
So what makes a smörgås a smörgås? The key component here is smör, meaning butter, which is spread on the bread before the other ingredients go on top. Gås means goose, and it's uncertain what these have got to do with Swedish sandwiches. Theories include the use of goose meat in early recipes, or a description of how butter may have looked during the churning process when people made it at home.
For a clue as to just how central smörgåsar are to the Swedish psyche, just look at how many idioms and other words they appear in. Smörgåsmat (sandwich food) is the term for usual sandwich toppings such as cold meats and cheeses, and instead of 'skimming/skipping stones' (throwing pebbles across a water's surface), Swedes kastar smörgås (literally 'throw a sandwich').
But there are also other words for 'sandwich', most commonly macka, thought to come from the Romani word mak (bread) or possibly the German verb machen (to make), which is used in almost the same way as smörgås but slightly more often when talking about toasted or heated sandwiches (varma mackor).
Han hade en smörgås i handen
He had a sandwich in his hand
Jag åt upp min smörgås
I ate up my sandwich