Rice vinegar is made from rice and it is sweeter, milder, and less acidic than Western vinegars. It is an essential ingredient in sushi rice and sunomono(vinegared salads). Also, rice vinegar is known for its anti-bacterial properties and this is why it is often used in Japanese dishes that include raw fish, seafood and meat.
Substitution: You can substitute with white wine vinegar/apple cider vinegar and add a little sugar (and water maybe) as rice vinegar is very mild. However, I would recommend you to buy rice vinegar because you really don’t want to use a Western vinegar that has strong vinegar taste.