A street library is usually little more than a small weatherproof box on a post outside someone's home, stocked with books free for anyone to take, and open to anyone willing to leave one behind in return, though even that exchange is rarely enforced with any real strictness.
What makes the idea work is its complete lack of friction: no membership, no due date, no obligation beyond simple goodwill, which lowers the barrier to reading for anyone passing by far below what even a well-run public library, with its forms and hours, can manage on its own.
Beyond the books themselves, a street library tends to become a small, informal landmark for a street, a known point of contact between neighbours who might otherwise never speak, built entirely around the shared, low-stakes act of leaving something useful for whoever happens to walk past it next. It is a small system that runs almost entirely on trust, and remarkably, that trust tends to hold up far better than most people would initially expect.