Hi PaulineD
Are you by any chance looking at a holiday which flies to the US?
ESTA stands for Electronic System for Travel Authorization and is required to travel to the US. In practical terms it is a reference number which the US authorities will have on their system when you arrive.
You can apply for this directly through the U.S. government's official web portal for all ESTA transactions (at least 72 hours before travel and is valid for two years), alternatively we can obtain one for you - which is why you will see it in the booking process.
I hope that information helps.
Kindest regards,
Julia