The proto-Indo-European language is like the grandparent of many languages that we use today. It was spoken a very long time ago, by people who lived in an area that is now called Europe and western Asia. This language is the oldest form of Indo-European language, which is a family of languages spoken by lots of people in Europe, India, and other places in the world.
Think of it like this: just like how you have a family with many cousins and relatives, languages have their own family tree too. Proto-Indo-European is like the great-great-great-great-grandparent of many of the languages we speak today, such as English, Spanish, German, French, Russian, Hindi, and even Persian.
Because this language was spoken so long ago, we don't have any recordings of it or people who still speak it today. But we can learn about it by studying the words and grammar of the languages that came from it.
For example, we can look at the word for "mother" in different Indo-European languages. In English, we say "mother", in Spanish it's "madre", in French it's "mère", in German it's "Mutter", and in Russian it's "мать" (pronounced "mat'"). These words are all very similar to each other, and experts believe they come from the same word in the proto-Indo-European language.
So, in short, the proto-Indo-European language is a very old language that is like the grandparent of many languages we use today. We can learn about it by looking at the words and grammar of current languages and tracing them back to their roots.