Step 1: What is Tollens' reagent.
Tollens' reagent is a clear solution that holds silver ions as $[\text{Ag(NH}_3)_2]^+$. It is used to test for aldehydes.
Step 2: What aldehydes do.
An aldehyde is easy to oxidise. When it meets Tollens' reagent, the aldehyde gives away electrons and turns into a carboxylic acid.
Step 3: What happens to silver.
The silver ions take those electrons and turn into silver metal. So $\text{Ag}^+$ becomes solid $\text{Ag}$.
Step 4: Where the silver goes.
The fresh silver metal sticks to the inside of the clean glass. It forms a shiny coating like a mirror.
Step 5: Name the result.
This shiny silver coating is called the silver mirror. It is the clear sign of a positive test.
Step 6: Final choice.
So an aldehyde with Tollens' reagent gives a silver mirror.\[ \boxed{\text{Silver mirror}} \]