From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
Fake Planet Namek is a planet that looks similar to Planet Namek.
Fake Planet Namek's disguised appearance had green sky and lakes like the real Planet Namek, but also deserts, forests, caves, and castles. Its true appearance was a dark and cloudy area full of man-eating monsters. Raiti and Zaacro were trapped on the planet. A fog around the planet made it inescapable unless the Stellar Winds were near.
In the manga, Son Gohan, Kuririn, and Bulma Brief went straight to Planet Namek upon their departure from Planet Earth. In the anime, however, the Dragon Ball Gang encountered two obstacles. The first was an invisible spaceship full of aliens who mistook them for the henchmen of Freeza. The second was Fake Planet Namek.
Upon landing on Fake Planet Namek, they were knocked unconscious. Raiti and Zaacro found them and took them to a building, creating illusions of Planet Namek after reading their minds with their antennas. Why the fake planet looked like Planet Namek on the outside when Raiti and Zaacro had not read their minds yet is unknown, but it might be a special feature of the planet itself.
Raiti and Zaacro disguised themselves as Namekians and constructed some quests to find the Dragon Balls. The first Dragon Ball was found by Raiti under a boulder in the middle of a lake. The second was within the mouth of a skull in a coffin in an underground cave. The next three were more of a challenge. The third was taken by a monster Dinosaur that fell into a lake full of acid forcing Gohan to go in with a specially designed suit and get the Dragon Ball, escaping the lake with a Kamehameha. The fourth was inside a tornado that Gohan jumped into it to grab the Dragon Ball and later destroyed with a Masenko. The fifth was inside a giant's castle above a forest where the forest floor came to life in the form of tree birds that took Krillin, Gohan, Bulma, and Zaacro (who takes Raiti's place at this time) to the castle. The Dragon Ball was on the giant's earring and they got it off, but unfortunately the giant woke up. Krillin and Gohan fought the giant, causing some considerable damage to him. (The giant was one of only four characters, the others being Freeza, Cell, and Kid Boo, to be hit directly by Krillin's Kienzan – although he might not count because he was an illusion). Despite losing a hand, he was still able to fight back until Bulma and Zaacro rescued Krillin and Gohan inside a toy airplane.
The sixth Dragon Ball was frozen inside an ice cave. Krillin and Gohan slowly melted the ice with energy waves to get the Dragon Ball, but the cave began to collapse (Raiti and Zaacro might have been the true reason behind it) so they sledded down the tunnel on a block of ice. Raiti and Zaacro sent a boulder after them, but they still escaped the tunnel. The seventh Dragon Ball was inside a building, but the man-eating monsters (outside of the illusion) attacked Gohan before he could get it. Raiti and Zaacro revealed their true forms and intentions, and the illusion disappeared. Bulma left to stall Raiti and Zaacro, who intended to steal their ship, until the Stellar Winds came so that they could escape. Just as they were inside, Krillin and Gohan, having dealt with the man-eating monsters, stopped them and knocked them down. As the Stellar Winds were almost gone, Krillin and Gohan did not have time to kill them, therefore they and Bulma got into the ship and left Fake Planet Namek through the Stellar Winds. Raiti and Zaacro were left on the planet, and presumably were still there at the end of Dragon Ball Z.