As others have said -- as soon as you get far enough along in the booking process (e.g., entering passenger details), it puts a hold on the fare. If you then exit out of the booking process, the hold is not immediately released. I believe there may be an associated record locator at that point as well that you can go back to . . . or just wait until the hold is released several hours later.

If the held fare is the last seat available in that fare class, you or the the next person who searches for the same routing will almost certainly pay more.