It is not a quantity. This is just a symbol to indicate the change. For example if use
![increment x](https://search-static.byjusweb.com/question-images/byjus/_61dd2a3c-8b12-45ec-9977-9eee6212abf420180807-28344-uk3zxt.svg)
it implies the change in te value of x. The symbol used for denoting change is triangle shaped. Actually this is the standard symbol. We use this symbol from recent years.