Step 1: Identify the idiom.
The idiom is "spilled the beans", used in the context of someone revealing information about their new car.
Step 2: Understand its figurative meaning.
"Spill the beans" means to disclose secret or private information, often unintentionally or prematurely.
Step 3: Select the correct meaning.
"To reveal a secret" is the correct interpretation. The other options (cook/cut beans, speak truth) are either literal or loosely related and do not reflect the standard idiomatic meaning.
\[ \boxed{\text{to reveal a secret}} \]