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.
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.
The Guardians also seem to have some measure of telepathic ability. Specifically, during the zhian'tara, the Guardians, acting as a conduit, via physical contact, telepathically transfer the memories of former hosts to volunteer bodies.
Xi was a Guardian in the 32nd century who provided aid to the young human Adira Tal, the first non-Trill host to a symbiont in more than two millennia. (DIS: "Forget Me Not") He later worked with Adira in a zhian'tara to transfer the essence of the Tal symbiont's immediate prior host, Adira's boyfriend Gray, into a golem body. Gray had also expressed a desire to become a Guardian himself at one point, and Xi also hoped he could be trained as a Guardian upon his return. (DIS: "Anomaly (DIS)", "Choose to Live")