Basically, leaked gameplay has revealed that when you land your ship on a random location of one of Starfield 1000+ planets and moons, instead of landing in one of the Bethesda hand designed settlements on those planets, the Creation engine renders a box around your ship, large enough to run for 10 minutes in any direction from your ship, and then when you run for 10 minutes you hit an invisible wall and get a pop-up box telling you to fast travel to your ship or too one of the other fast travel points on that planet/moon, such as settlement if the planet/moon has one. 

The randomly generated planets are still fully explorable, you just have to fast travel back to your ship, pick a new landing zone, and then explore within that 10 minute radius box around your ship. Basically No Man's Sky fans were hoping that you could land, pick a direction, and run for hours and hours I guess. But Starfield isn't No Man's Sky, it was never intended to be. Most of these planets exist primarily for light exploration, resource gathering/mineral extraction within the planet surveying checklist, building a base on any planet in any location you choose, and as modding canvases. You don't need to run more than 10 minutes in any direction, if the planet survey tells you that a mineral you need is within the bounds of that landing zone, then you can find it there without needing to run further. 10 minutes from your landing spot is plenty of space to find somewhere to build your base. 

The meat of Starfield is the handcrafted content, the settlements, the quests, the NPC's, not the exploration of the barren areas of planets. The planet exploration stuff is more of a bonus, it's not intended to be the meat of the game. Those who want full, proper exploration of huge planets already have other games for that, including No Man's Sky. The limitation likely exists due to Creation engine, this 10 minute barrier is likely why there are no ground exploration vehicles such as wheeled or hover vehicles, or in-atmosphere spaceship flight. Who knows, maybe one day modders will find a way around this limitation of Creation engine. But in the meantime, I hardly feel those things are necessary, when most of the actual content in the game is in the hand designed settlements, not in the randomly generated barren areas.