The Lockean Proviso is a way of thinking about fairness when it comes to having and using something. Let's say you wanted to build a fort in your backyard with some blocks you found in your playroom. You can use the blocks to build a nice fort, BUT, you need to think about the Lockean Proviso. That means you need to think if it's fair for you to use all the blocks to build your fort if other kids in the neighborhood can't also use blocks to make their own fort.
So, the Lockean Proviso is a way of making sure that everyone has a fair chance to have what they need and use it if they want to. That means you need to make sure that there are enough blocks in the playroom for everyone to use if they want to build a fort.