A reality is a stack of planes, like sheets of paper. Every sheet is made of the three core planes; the balance between them is what changes from sheet to sheet.
The Prime Material Plane is the most central sheet, usually the most balanced. Everything else is described relative to it.
Push the balance past the middle toward any one core plane and physics changes drastically. A being whose footing is strong in the other core planes struggles to keep existing there — possibly to the point of ending.
Lesser planes — every sheet other than the core three — can be physically travelled to. You can exist in more than one at a time, because the core planes run through all of them.