The UEFA Nations League is a biennial international
association football competition contested by the senior
men's national teams of the member associations of UEFA , the sport's European governing body.

The first tournament began in September 2018, following the
2018 FIFA World Cup . The four group winners from League A will qualify for the finals, to be played in a yet to be decided European city in June 2019. Four nations, one from each League, will also qualify for the UEFA Euro 2020 finals.
The competition will largely replace international friendly matches currently played on the FIFA International Match

Adoption

In October 2013, Norwegian Football Association President
Yngve Hallén confirmed that talks had been held to create a third full national-team international tournament for UEFA members in addition to the FIFA World Cup and UEFA European Championship .

The concept of the UEFA Nations League would see all 55 of UEFA's member associations' national teams divided into a series of groups based upon a ranking formulated using their recent results, where they would be promoted and relegated to other groups according to their results within the group. The proposed tournament would take place on dates on the International match calendar that are currently allocated for international friendlies and would not affect the FIFA World Cup or UEFA European Championship.


Format

According to the approved format (prior to Kosovo becoming UEFA member),the now 55 UEFA national teams (including Kosovo) will be divided into four divisions (called "Leagues" )
12 teams in League A, 12 teams in League B, 15 teams in League C, and 16 teams in League D. In each league, four groups are formed (three or four teams in each group) and play each other both home and away.

In the top league, League A, the winners of the four groups go on to play in the Nations League Finals, with two semi finals and one final to decide which team becomes the UEFA Nations League champion.
Teams can also be promoted and relegated to a higher or lower league. Each group winner (there are four groups in each league) is automatically promoted to the next higher league for the next tournament. Each team placing last in its group is automatically relegated to the next lower league.


https://en.m.wikipedia.org/wiki/UEFA_Nations_League

this stuff is so boring, I see it being discontinued sooner rather than later