From the AA site.

Need more time to decide?
We offer convenient hold options to assist with your travel planning.

24 hour hold
When available on select American Airlines marketed and operated flights, you can hold your reservation up to 24 hours for free if you book seven or more days prior to departure.

Here’s how:
Choose your flights and select “hold” on the “Review and Pay” page
Refer to your confirmation email or visit “My Trips” to pay for your trip
Your reservation will automatically cancel after 24 hours if you don’t complete the purchase