Travel Agents always have weird codes and abbreviations that show up on their records/systems. While it might well be whatever she says it is, ESTAs are also the system that is replacing the Visa Waiver Program.
You do need to fill out an online form and gain authorization from the American government before you will be permitted to enter the US. This MUST be completed before you travel. This system is now mandatory for anyone traveling to the US.
Even if the ESTA is filled in, immigration can still refuse you entry.