Could someone please explain the union to me. I don't understand. I get the intersection ones but not union. HElp me?

Ecample: (-1,1)U[0,6]

intersection is where the intervals overlap

union is evetrything in either one

(-1,1) extends from -1 to 1
[0,6] extends from 0 to 6

If you supermimpose the two shaded intervals, you will see that the combined shaded interval is

(-1,6]

The stuff in [0,1) is shaded twice, but that's ok. It's part of everything that's shaded.