The Trill Guardians were a group of unjoined Trills, dedicated to the care of the Trill symbionts.
The Guardians worked under the auspices of the Symbiosis Commission. They served the symbionts by maintaining the underground pools that nurture them, such as those in the Caves of Mak'ala on Trill. Some duties included monitoring the ion concentration, temperature and viscosity.
Timor was a Trill Guardian in the 2370s. (DS9: "Equilibrium")
The Guardians also served the well-being of the symbionts after they were joined, and helped the new host adjust to preserve that well-being.
During the zhian'tara, the Guardians telepathically transferred the memories of former hosts to volunteer bodies.
Lela Dax found the Guardians to be "odd fellows." (DS9: "Facets")
In 2375, Benjamin Sisko suggested to Ezri Dax that she could become a Guardian when she considered leaving Starfleet. (DS9: "Afterimage")