Pour cold water on : This phrase is an idiom. It means to discourage someone. When somebody tells you about his plan to do something interesting,you pass a negative remark about his plans, you 'pour cold water on his plan.
For example: Mary's grandfather has not approved of her plans to join the army.He has poured cold water on her dreams.
The little girl is planning to become a singer. Please encourage her and don't pour cold water on her plans.