Data augmentation works by applying various transformations to your training images (or other data types) that preserve the core content while changing superficial aspects. For image data, common augmentations include:
In fastai, data augmentation is implemented through the aug_transforms() function, which you can include in your DataBlock:
data = DataBlock(
blocks=(ImageBlock, CategoryBlock),
get_items=get_image_files,
splitter=RandomSplitter(),
get_y=parent_label,
item_tfms=Resize(224),
batch_tfms=aug_transforms()
)