Not necessarily, sometimes dimensionality reduction (the more common terminology used, for what is basically compression) is the explicit goal.
Can be used for outlier detection, similarity search, etc.
During training, you find a projection of the input, for example an image, to a smaller space, and then back to the original image. This is referred to as encoding and decoding. The error fuction would be a measure of how similar the in- and output images are.
It took me 30 years to learn that change is possible