Indo-European languages are a group of languages spoken by people in many parts of the world. The idea behind the Indo-European substrate hypothesis is that the languages we speak today are influenced by the languages spoken in the past by people who lived in the same areas.
Imagine you have a toy box with many different toys. Each toy represents a different language. Now, picture that the toys all belong to different children who played with them before you came along. They may have left some of their own toys mixed up with yours. Some of those toys may have affected the way you played with your own toys.
Similarly, when people migrate to new areas, they bring their own languages with them. Sometimes, when they meet other people, they learn new words and ways of speaking. This is called cultural exchange. The Indo-European peoples migrated widely, so we can see many traces of their various languages in one another’s.
To better understand this, imagine you are playing with your toy box and your friend comes over. They have a toy box too, but their box has different toys than yours. You may learn new games to play and use new words to describe your toys.
These cultural exchanges can also happen over time. Imagine you have been playing with your toy box for a long time, and over time, you make new friends who also bring different toys to play with. Eventually, you will have acquired many new toys and have adapted your play style to include new games and new words to describe things.
This is similar to what happened with Indo-European languages. As groups migrated to new areas, they mixed with other peoples and gradually adapted their own languages to reflect these changes. The Indo-European substrate hypothesis helps us understand how these influences shaped the languages we speak today.