That's a marcato, indicating that this note/chord needs to be played much louder than the surrounding notes, even louder than with a more common sforzando accent (the wedge pointing to the right). (left: marcato, right: sforzando)

What does the triangle symbol mean in music?

It means "major 7th", not just "major". So, you see "∆ " on a minor chord sometimes. "Am∆" = A C E G#. It may not mean the major 7th has to be included (although 99% of the time it would be).

What is the upside down triangle in music?

For extremely short notes, a mark that looks like a triangular wedge or exclamation point without the dot is called a staccatissimo. It's even shorter and edgier than staccato.