A deliberately minimal file transfer protocol with no authentication at all. Used for network booting, and for pushing firmware and configuration to switches, routers and phones.
No authentication, no encryption. Acceptable on an isolated management network and nowhere else. Exposed TFTP has leaked router configurations — including credentials — many times over.
An open TFTP port is a serious finding. It has no authentication, so anyone who can reach it can potentially read the configuration files it serves. Restrict it to an isolated management VLAN or turn it off.
The port scanner will tell you whether 69 is reachable on a host you control. Checking from outside your own network matters: a port can be open on the machine and still be blocked at the firewall, and it is the view from the internet that decides whether anyone else can reach it.
Only scan hosts you own or have permission to test.