Can someone break down exactly what “Interdimensional” would mean?

I know the textbook definition. I understand the basics that it could be parallel planes, different dimensions, or even different times. But I’m having trouble really grasping what it would mean. Can someone give an “ELI5” explanation that could be given to others who aren’t into this stuff? And even though I understand what it means (in a definition sense), what does it mean in a practical sense?

And if these are “interdimensional”, what does that really mean for us as humans living today?