A spray wall is a wall where a tonne of holds are bolted on without any specific routes/colors being set. Usually the holds fill basically every bolt hole (similar to Korean and Japanese climbing walls) rather than a more spaced out set. Spray walls are excellent for training as they are used to make up your own routes - normally to train specific moves or to recreate outside climbs. Sometimes there will be a folder with a photo/sketch of the wall and the holds with made up climbs or ones to set for yourself and train. They're also great for training with friends who tell you what holds to use aka "Addon" or "Nominate" games. Some spray walls are set with wooden holds for hands and tiny metal or plastic holds that are meant for feet only. Wooden holds are easier on the skin but less grippy so are great for higher level training over a longer period. Sometimes the holds are set mirrored so you can check or train specific sides.