Okay kiddo, so imagine you want to buy a toy at the store. The person selling the toy cannot ask you what your favorite color is or what your favorite food is before letting you buy the toy. That's kind of like the "no religious test" clause.
It means that if someone wants to work for the government or be in a position of power, other people can't ask them about their religion before allowing them to be in that position. Just like the toy seller can't ask you about your favorite color before letting you buy the toy.
Does that make sense, little one?