A more thorough explanation:

A counterfoil is a detachable part of a writing that summarizes the important details of the main part. The most common example of a counterfoil is a check stub. On a check stub, you can find the date, the name of the person or company receiving the payment (payee), and the amount of money being paid.

• When you write a check to pay your rent, you tear off the counterfoil and keep it for your records. The counterfoil shows the date you wrote the check, the name of your landlord (payee), and the amount of rent you paid.
• If you buy tickets to a concert online, you might receive a confirmation email with a counterfoil attached. The counterfoil will summarize the details of your purchase, such as the date and time of the concert, the name of the artist, and the price of the tickets.

These examples illustrate how a counterfoil is a useful way to keep track of important information. By summarizing the details of a transaction or agreement, a counterfoil can help you remember what happened and provide proof of payment or purchase.