Old Spanish language is a way of talking that people in Spain used a long, long time ago. It's kinda like how you might talk differently from your grandparents or great-grandparents.
Back then, Spanish language had a lot of differences from the way we speak it now. For example, they used different words and the grammar was different too.
Sometimes, it might be hard to understand what people back then were saying because their language was so different. But just like how you might have trouble understanding a different language now, people in the past probably had no problem understanding each other.
Overall, Old Spanish language is just a way of talking that people in Spain used to use a long time ago that might sound different from how we speak Spanish now.