There is a lot of information not mentioned about your thyroid ultrasound, including if any nodules were noted or the dimensions of the thyroid gland itself, which can tell you more about your thyroid. I'm not a medical professional but I have had thyroid ultrasounds, so I'll tell you what I know based on the short ultrasound result.
As for what symmetric mild parenchymal heterogeneity means:
Parenchyma is "the functional tissue of an organ as distinguished from the connective and supporting tissue", so parenchymal is referring to the normal tissue of the thyroid gland.
Symmetric in this case I believe is referring to both lobes of the thyroid.
Heterogeneity means the tissue does not look the same throughout, some places it might be darker, some lighter on the ultrasound. Homogeneous tissue looks the same throughout. Heterogeneity of the thyroid tissue as opposed to homogeneity can sometimes indicate thyroiditis or inflammation of the thyroid, usually caused by autoimmune disease such as Hashimoto's or Graves disease. Your ultrasound indicates mild heterogeneity, so I wouldn't assume you have inflammation until you hear more from your doctor. (Again, this is where size is useful because an enlarged thyroid plus heterogeneity often indicates inflammation due to an autoimmune response, but hard to tell with the description of "mild heterogeneity")
So, symmetric mild parenchymal heterogeneity could mean that both lobes of the thyroid are showing some heterogeneity, which could be an early indicator of thyroid inflammation, but since it is mild, I wouldn't assume anything is wrong at this point, unless you have had additional tests other than the ultrasound that indicate a thyroid problem.
Have you had any thyroid blood tests? Either hormone (TSH, free T4, free T3) or antibodies tests including thyroid peroxidase (TPO), Thyroglobulin antibody (Tg), and/or Thyroid stimulating immunoglobulin (TSI)? Do you have hypothyroidism or hyperthyroidism symptoms? All of these are necessary if you are trying to diagnose Hashimoto's or Graves disease. It's unclear why you had an ultrasound in the first place, but if you are having hypo or hyper symptoms it could be an indicator of an autoimmune response, but more tests are needed.
(I had heterogeneity with diffuse nodularity on my ultrasound, and I wasn't diagnosed with Hashimoto's until my antibody tests came back High for Tg and TPO.)