What does abnormal uptake on a thyroid nuclear scan mean?

Several things. The result of a thyroid uptake test could be abnormally increased or decreased. In the usa, the normal thyroid uptake range is approximately 10-30% at 24 hours. An increased result may be due to various reasons including: grave's disease, multinodular toxic goiter or hashitoxicosis. A decreased uptake could be due to subacute thyroiditis, iodine or amiodarone induced thyrotoxicosis among others.
Various possibilitie. Thyroid uptake must be interpreted with the results of some blood tests such as TSH , T3 (liothyronine) and T4 values and the clinical context. Usually high uptake ( more than 30%) is related to hyperthyroidism ( graves disease, thyroiditis..) and decreased uptake to hypothyroidism. However, there are various possibilities and all these data should be interpreted together. You should ask your physician.
Low or high? Low uptake could indicate hypothyroidism, while high uptake might indicate hyperthyroidism (grave's disease). The test (scan) you had would have also detected a hot nodule, which would need to be surgically removed. But it does not appear by your question that this is of concern.
See below. High uptake suggest hyperthyroidism and low uptake may suggest hypothyroidism and its multiple variations. See your doctor to discuss results of the test.