In general, the term, 'what is' is seldom used verbally and in letters. However, you will meet this term very frequently here in my blog and so I think I have to make the meaning of 'what is' understood obviously.
Even though I said I had to explain about the meaning clearly above, most of the readers must be aware of the meaning. When we call or express a thing verbally or in letters, we have some different words for it. For example, we have the terms such as a friend, a companion, a buddy, a pal, a mate, etc. 'What is' is something that actually is or exists, and so it can also be expressed as a thing which exists, things which exist, a fact or a truth that exists, facts or truths which exist, what happened, what has happened, etc.
If there is anybody that doesn't have the clear understanding of it, e-mail me here, firstname.lastname@example.org or email@example.com.