A few excerpts from Ra to describe it :

“Where's your personality Sun-Earth? Are they in something that's defined? Are they part of a defined center? Is that defined center part of a continuity that leads to your inner authority? For most people, the answer is no.“

Sun in undefined center : “There's these people that will have their Sun in an open center, that's like living in London truly, you know the sun never shines. You run around all the time saying “I've got to move! How to Move? I gotta get this, somebody, please let my light out.”

Earth in undefined center : “And then you have these people whose Sun is in a defined center, and their Earth is in an open center, you know, and it's like it's all over the place. But nothing's grounded. And when you're talking about the personality, when the Personality is not grounded, we know what happens.”

“Having your Personality Sun Gate in an undefined center is like having the Sun in a closet. Nobody sees it. And you are always looking to hook it up.”

“Not being grounded by our Earth doesn’t necessarily feel like not being grounded emotionally or energetically at all, it’s the energy of the Sun that is not grounded by the energy of our Earth. There are many ways to experience this. I experience it more as a restlessness when it doesn’t happen for a long time and some peace brought out by my Earth being connected again. This is harder to catch when it happens in the body (Design - red - “unconscious”) imprint, but it means also that it’s often less daunting to experience than on the Personality/mind side.”

“Whenever you see the sun, either conscious or unconscious, in an undefined center, it is as though this person's light is locked up in a room with all the shutters closed. This can create all kinds of problems in life for such people.

The reason for this is because these people cannot find consistent access to 70% of who they are. They are dependant on others and on planetary conditioning to define the gate. Thus the seduction of the harmonic gate is huge, because that person hooks them up to their inner light. - unsure Probably R. Rudd

“When you have your Sun, and in this case, both the Sun and the Earth, Personality Sun-Earth, in an open center, you’re always dependent on others. You have to be. I don’t mean dependency. I don’t mean dependent in that way. I just mean that they are the essential allies in their process. And that it’s the gods, if I can put it that way, saying, ―Hey, look you’re not supposed to be alone here anyway, just be correct in your decision making.

But it also says something very profound about that. Those beings that have their Personality Sun-Earth in open centers actually gain an enormous amount of wisdom about the very nature of their Personality. And they do that because they see their Personality lived out through the input of the others that are in their aura, through that veil of the amplified emotionality in this case. “

Another, longer quote lives here : https://thehappymessproject.com/human-design-quotes-bank-1/cross-gates-in-undefined-centers-some-crosses-dont-have-a-full-time-job