When you add an instance of a block onto your interface, you may want to customise it or alter it's styling.
You can add style rules that apply only within the scope of the selected instance of a block. To add styles select a block, and expand the CSS stacker drawer. Any CSS you add here will apply only to the selected block itself and not affect any other elements in your interface. As it's scoped you also do not need to write a selector rule, you can simply write the CSS entries and they will apply to the block and it's contents.
For example: If you select a block and you want the contents of that block to be aligned to the center, you can select the block and add text-align: center; without having to write .my-block { text-align:center; }