Planar Travel

The barrier between dimensions (or planes of existence) is permeable. Things pass back and forth through it. Sometimes physical, sometimes something other. This other can be information, in the form of communication, energy as a fuel for spells or even raw magic, the essence of change itself.

Many forms of magic are designed to pierce the veil. Certain versions of fireball don't actually generate fire but instead a tiny collapsing portal to the fire plane at about two hundred meters below the surface ocean of flames.

Natural portals sometimes form through which the curious might stumble but these are rarely permanent as the planes are often moving around and reconnecting. Mapping the place and timing of these portals would be the work of a lifetime and is considered a highly dangerous profession, but also extremely valuable to arcanists and adventurers who can only find what they need in other planes.

Some powerful beings make their homes through shifting portals. Perhaps an ancient Faye awaits on the other side to test mortals in exchange for a boon, perhaps a demon waits to hunt on moonless nights while safe for the rest of the month.

Access & Availability

Actual travel between the planes is not easily accessed. Only powerful magic-users or mystical creatures can force a tear in reality. Otherwise one must find a naturally occuring portal.

Complexity

Reasonably complex, it is also a matter of balancing power through the complex framework, to avoid collapsing it. One could attempt to do it with less power, but it would be considerably less stable.