The father tongue hypothesis is a theory that says children learn language from their father, or the male figure in their life, more than they learn it from their mother or female figure. It's like when you learn how to do something from your dad or grandpa, like tying your shoes or throwing a ball.
Usually, children spend more time with their mom or female guardian, so people thought that they learn language more from them. However, some researchers found that children with a strong bond with their dad or male figure tend to have better language development.
This doesn't mean that moms or female figures aren't important in teaching children language. Both parents or caregivers play important roles in helping children learn how to talk and communicate. It just means that dads or male figures can have a bigger impact on a child's language development than people originally thought.