What do they do for a cyst on kidney?

Mostly nothing: In majority of cases nothing, since most of these cysts are benign. They have to be closely monitored (periodic sono/ct scans).

Renal cysts: Renal cysts are quite common and are most frequently without any symptoms. As a result they are most often detected as an incidental finding on imaging done for unrelated symptoms. Simple cysts require nothing further. Complex cysts are monitored with annual imaging. Cysts with a solid component may require partial or complete removal of the kidney due to malignant potential.

Fluid : It is a fluid filled structure which most of the time is of benign nature. When they are multiple they require further evaluation for not so benign conditions. On the least occasions they could be associated to a malignant structure and require extensive tests and treatment. A follow up with urology is recommended in order to decide the nature of the cyst/ cysts.