A custom block style is a style change that is applied to a WordPress block to make it look different from the default styles.
The difference between a custom block style and a custom block is that the style is added to existing blocks, while custom blocks are new blocks. It is usually simpler to create custom styles than entire blocks.
The block style can then be selected in the WordPress editor:
How a custom style is added
If you want to know more, you can continue with these two guides: