(Random) You shuffle the deck every time you go through it. You're forced to learn the images and their classifications without relying on any specific sequence, as the data has no signal from sequence order.
(Fixed order) Every time you go through the deck, the images appear in the exact same order. Over time you may start to unconsciously memorize the sequence of flashcards, rather than the actual classification of each image.
When it comes to actually training a model, if the batches are sampled sequentially from a dataset, it risks learning from correlations caused by the sequencing of the data, resulting in poor generalization. In contrast, when you sample the batches randomly, the model is biased and encouraged to learn features from the data itself rather than from any signals that arise from artifacts of the ordering.
