From the Super Mario Wiki
Subspace is an alternate, but possibly parallel dimension to Subcon that turns everything dark and reverses direction. It is a single screen which the player cannot travel beyond. Subspace can only be accessed by use of the Magic Potions (which create doors leading to the land) in Super Mario Bros. 2. When Mario or his friends enter Subspace, they only have a few seconds to get the Mushroom that may or may not appear; it depends on where in Subspace the hero has entered. In Subspace, all items found in the red grass are transformed into Gold Coins when the player plucks them, giving the heroes an increased chance of earning extra lives in the slot machine bonus game. Depending on the area, players can also collect Cherries, Stars, and POW Blocks. A few Vases allow the player to skip to another world like a Warp Zone. There are no enemies, but Phanto can follow the player into Subspace if he or she was holding the Key. The player can still die by falling into the abyss or drowning in quicksand. The music playing in the background is the main theme from Super Mario Bros.. In both remakes, the background is the same, but the level actually has color and is not just simply black. Additionally, in the GBA remake, Yoshi Eggs could be found here after Wart is defeated.