A lashing device is a rope, wire cable, or strap which wraps around the board and essentially ties it up with some sort of locking device. A simple low cost way to do this is to wrap chain around the surfboard and secure it with a lock.
The passage describes a lashing device as a way to secure a surfboard using a rope, wire cable, or strap that is secured by a locking mechanism. It provides an example of how to do this using chain and a lock.