system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Stephen Hawking's "No-Boundary Condition"
gpt
Think of a string on a guitar. It is attached to the guitar on both ends, and this attachment constrains the ways in which the string can vibrate and produce sound. (Only certain pitches can be produced for a given length and tension, for instance.) If you detach one end, even if somehow you could maintain the same tension on the string, its behavior would change and many other pitches become possible.

If the string represents our physical universe, having "attachments" at its physical endpoints (called boundary conditions), or a specific configuration at an initial zero point in time (e.g. a birth of the universe) would constrain the laws of physics within the universe in a way that would differ from the laws of physics "outside" the universe. 

Having no boundary conditions, as Hawking asserts, would imply that either the universe is an open system with no boundary in either space or time (no beginning, end, or borders), or that if those boundaries did exist, there would be no information about the state of the universe at those boundaries to constrain the physical laws within it, like a string which is unattached to its guitar.

Either way, that implies that the laws of physics apply equally within the universe as they do outside of it.