ШТО и ШТА depend of the context of the sentence. ШТО is a short version of ЗАШТО. ШТА can be a question for the subject, but also can be a question, but not a subject. Example: Шта радиш? Шта гледаш? Шта читаш? ... You're example Што је то? is Croatian and Шта је то? is in Serbian. Sentences are the same, but in different languages.

што could be WHY ( zašto) and šta is hat.

Hi Ruslan! In Russian or in Serbian?

Hello, "Shta" isjust incorrect and slang unofficial form of "Chto" that was created in memes. Young people use it in case when one's surprised and he/she asks "Shta???"