ALPN (Application-Layer Protocol Negotiation) is a TLS extension used for determining which application protocol (like HTTP/2 or HTTP/3) will be used within a secure connection. It's crucial for optimizing web connections because it allows the client and server to agree on the protocol during the initial TLS handshake, rather than needing additional round-trips. This reduces latency and improves the efficiency of secure connections, particularly beneficial for performance-critical applications.