Proto-Semitic language is a way that people used to talk a very long time ago, before we even started writing things down. It's like how your parents or grandparents may have talked differently than how you talk now, but even more different because it's from a really long time ago.
It was a language that was spoken by a group of people who lived in a place called the Near East, which is now parts of countries like Iraq, Syria, and Israel. These people were the ancestors of many groups of people who still live in that area today, like the Arabs and the Jews.
But because it was so long ago, we can't hear or see what they actually said or wrote. Instead, experts try to figure it out by studying words from the languages that came after it. So, they look at words in Arabic and Hebrew (which are some of the languages spoken in that area now) and try to trace them back to their earliest form, which is Proto-Semitic.
It's kind of like figuring out where you came from by looking at your family tree and seeing where your mom and dad and grandparents came from. People who study languages use things like that to figure out where words and languages came from way back in time.