Referendum (folkomröstning) on a single issue
What A vote where the voters get to say their view directly on a single issue, instead of the issue being decided only by those elected.
Who You yourself vote, but it is the politicians who decide to hold the vote, and at the national level the result is usually advisory.
How When a referendum has been called you vote as usual, and you can follow and take part in the debate about the issue beforehand.