As the name suggests, Reusable Blocks are Blocks that you can re-use anywhere on your site. A Reusable block can be added to any Page or Post, and as many times as you want. If you update or change the style of your Reusable block, the changes are automatically applied to that block, right across your whole site.
To create a Reusable Block, simply add a block, add your content to the block then style it how you'd like it to appear. Once you've got the block looking how you want it, click the Add to Reusable Blocks option under the More options icon on the Block Toolbar. You're then prompted to enter a name for your Reusable Block. After doing so, click the Save button to save it. Your new Reusable Block will appear under the Reusable section within the Block Inserter.