Girl meets Boy.
Girl and Boy immediately have an attraction and either start a relationship or do a dance around their feelings.
Then another Boy enters the picture.
Girl finds herself attracted and torn between her feelings for the two boys.
And we the reader have to painfully endure all of the characters go back and forth with their feelings or this three-way relationship makes a mess of both their lives and the story that is being told.
And that my fellow readers is what is called a love triangle.