Proto-Turkic language is the very first language that Turkic people spoke a long, long time ago. It's like how when you were born, you didn't know how to talk yet, but then you starting learning and practicing and eventually you could talk with your family and friends.
Proto-Turkic language is special because it is the ancestor of many modern day Turkic languages, like Turkish, Uzbek, Kazakh, and more. It's like how you have a family tree that goes back to your great-great grandparents, and they had children, who had children, who had children, until eventually your parents had you. Proto-Turkic is like the great-great grandparent of all these languages.
Even though we don't have any written records of Proto-Turkic, linguists and researchers have been able to figure out what it might have sounded like and some of the words it used. They can do this by studying the similarities and differences between modern day Turkic languages and looking at older documents written in related languages.
So basically, Proto-Turkic language is the very first language that Turkic people spoke a long time ago, and it is like the ancestor of many modern day Turkic languages. Even though we don't have any written records of it, people have been able to study and figure out what it might have been like.