If you are making a default Bella to see how it fits we recommend a fabric such as Calico (Muslin) or a cheaper fabric that matches the drape and stretch of the fabric you intend to make a finalised version of Bella with.
You can use scraps of fabrics from your stash so don't worry about buying fabric specifically for Bella.
A block is a basic shape on which other patterns are based. They are sometimes also called slopers, although purists will argue that a block and a sloper are different things.
Blocks are typically not made as-is but rather serve as a basis for other patterns.