You can set a which is automatically displayed in a field unless the user enters something different.

Place holder

Explanation:

In computer programming, placeholder is a word, character or series of characters that is used to take up the space until the time when the space is actually needed. It is widely used in the sign up or login forms for web and mobile applications. For example, First Name, Last Name etc.

A programmer might have an idea that he will require this number of characters or values but he don't knows what the input will be, that’s why he uses the place holder.

Hope it helps!