Auto Enrollment Plan

No, it's not a plan to create a car that you can roll up like a sleeping bag. Instead, it has to do with retirement savings.
The concept is relatively simple, once its broken down. "Auto" is short for "automatic." "Enrollment" refers to signing up for (you guessed it) a retirement plan (which, by the way, provides the "plan" part). So an "auto enrollment plan" is a retirement program where an employer automatically deducts money from an employee's paycheck and puts it into a retirement account, such as an IRA or a 401(k).
The employee can choose to opt out of the program, but if they don't say anything, they will automatically get enrolled.
The general motivation for these programs: people get lazy about retirement. If you leave it up to employees to actively sign up for a plan - go into HR, get the paperwork, fill it out, then bring it back by whatever deadline - they probably will end up binge watching Game of Thrones instead.
So instead of making them do work to get into the program, the employer makes them do work to get out of it. A vast number of people will stay in the program from there, meaning their retirement accounts could be growing even while they watch Ned Stark get his head lopped off.