Mri shows that i have a cystic nodule of left thyroid lobe and a perineural tarlov cyst in left s2 (racepinephrine) neural foramen. are these related? what are they?

Tarlov cysts: Are an overwhelmingly unimportant finding so don't worry about it. The thyroid nodule is also most likely insignificant but ultrasound may be useful to take a second look so ask your clinician. The great majority of thyroid nodules are not cancerous.

Not related: Tarlov cyst is essentially an incidental finding, and represent fluid filled sacs/dilations of the the nerve root sleeves in the sacrum, and are an uncommon cause of back pain. A cystic nodule in the thyroid implies a nodule that has a cystic portion. You do not provide a size, but thyroid nodules are very common. Further workup, if any, should be based on your situation - talk to your doctor.