These are timers created for Charr's Timer Module (discord link) by QuitarHero.
These timers provide markers, trails, and timed warnings and alerts for Raids and Dungeons. Some bosses have multiple timers for specific purposes, while others only have a single timer. Some timers are still WIP and must be started manually with either Left Alt or Right Alt.
Timers are not dynamic, and can only be created and used for static, timed, and invariable events, such as Slothasor's Mushroom Spawns and Dhuum's Greens. They can not be reliably created for events such as Sabetha's Flamewall which can have its 45s interval interfered by one of the three minibosses. Regardless, all automatic timers start when you enter combat (not necessarily on fight start!).
For more information, check the Wiki.
Support Discord: https://discord.gg/rCEuzGz