Imagine you have a toy car that runs on batteries. When you put new batteries in the car it can go really far, but when the batteries start to run out the car can't go as far.
Now, instead of a toy car, imagine you have a real car that runs on electricity instead of gas. The all-electric range is how far this car can go on a single charge of its battery. So just like with the toy car, when the battery is full the car can go really far, but as the battery gets used up the car can't go as far.
So, when people talk about the all-electric range of a car they are talking about how many miles that car can travel on a single charge of its battery. It's important to know how far a car can go on a single charge so you can plan your trips and not get stuck somewhere without any power left in your car.