That’s an example of the optical telegraph the article is about. A person inside uses levers to move those arms into different positions. Each position has a meaning, similar to naval flags. Other towers in visual range can then re-transmit the message to towers further down range.

Same as clacks in Discworld – https://discworld.fandom.com/wiki/Clacks

edit: here’s a wikipedia with more examples of different semaphor towers https://en.wikipedia.org/wiki/Optical_telegraph