From Inkipedia, the Splatoon wiki
Ground is the default terrain in the Splatoon series.
The main ground type is concrete, though wood is also used such as large boxes, crates, planks, and logs. Ground can sometimes not appear if you glitched out or hack the game, and it is not seen or available in water, air, dirt, or pits. Ground is what builds up the game, and can be slightly changed in game in ranked mode.
Standard ground can be inked by Inklings to cover turf. Covering the ground is the primary objective of Turf Wars. Ground can be both walked on in human form, or swam in after covering it with ink in swim form, or swam on even without ink in swim form.
An example of concrete ground in Ancho-V Games.