Each content piece in the Block Editor is a block. Think of them as building blocks.
Blocks can be assembled and combined to create rich, detailed pages. Most of the blocks in the block library are WordPress core blocks, but some are unique to this theme or created by third-party plugins. Blocks get default styles from the theme, but their appearance can be changed using the block editor.
Resources to learn more about blocks:
Learn WordPress Lesson: Nesting and using blocks to create visually appealing content