What it's for
Port 69 is the default port for TFTP (Trivial File Transfer Protocol), a deliberately minimal file-transfer protocol with no authentication and a tiny command set. It's mostly used today for booting network devices (PXE boot, loading firmware onto routers/switches, provisioning VoIP phones) rather than general-purpose file transfer, since its simplicity makes it easy to implement in constrained boot-time environments.
Protocol basics
TFTP uses UDP, trading reliability for simplicity — the protocol implements its own basic acknowledgment and retransmission scheme on top of UDP rather than relying on TCP. There's no login step and no directory browsing; a client simply requests a specific file by name and the server sends it in fixed-size blocks. Because there's no authentication at all, TFTP should only ever be used on trusted, isolated networks (like a dedicated provisioning VLAN), never exposed to the open internet.