Ground truth image refers to actual image. Say for example if we focus to segmentation, then ground truth image tend to actual region of interest which should be segmented out of whole image. So that you can get idea that which part to be segment.
In short ground truth images are generally used for deciding how accurately your algorithm has worked,
how to get ground truth images? is this available online? i have tried searching many sites but i am not getting such image datasets having 1. MRI scanned images and 2. Tumor detected with its area shown.( ground truth images).