One of the most important steps in assembling an inflatable swimming pool in your backyard is ensuring it sits level on the ground. If it's crooked, the water is more likely to spill over one side and cause excessive damage to your lawn. Plywood can serve as a level foundation for a swimming pool.
Unless your yard is perfectly flat, it's important to level the area before you put down the plywood for the inflatable pool. Placing the plywood directly on an uneven yard is not ideal. Instead, use sand or dirt to build up indentations in the yard until the area is level. Pack the sand or dirt down firmly so it provides a solid foundation for the plywood.
It's costly to buy enough plywood to place directly beneath the entire pool. You also create the risk of a wood splinter puncturing the pool liner. Instead, measure the size of the pool's leg supports and cut plywood pieces that can sit directly under the legs. This way, one sheet of plywood creates enough wood for the pool.
Depending on the quality of the plywood you purchase, it might be smooth or roughly finished. Smooth plywood is more costly; if you would rather purchase low-end plywood, sand it smooth with a random-orbit sander or belt sander. The plywood doesn't have to be pristine, but it should be smooth enough that it doesn't contain splinters that could damage the pool.
Small pools that do not have leg supports around their perimeter do not require plywood to create a level base. It's common to situate these pools on level ground since they don't take up significant space. If the yard is not level, place sand or dirt in indentations to create a level area.