I believe they mean that the random variables have a common support. For example, if X ~ Unif[0,2] and Y ~ Unif[1,3], then their respective PDF's overlap in the interval [1,2], meaning the only interval where both X and Y can have common values is [1,2]. To illustrate, below is a representation of the supports of X and Y, below which is a representation of the real line.
1 Thyroid cancer is a malignant tumor of the thyroid gland. 2  Symptoms of thyroid cancer include swelling in the neck (goiter), neck pain, hoarseness, swallowing and breathing problem, swollen lymph gland in the neck.
Swelling of the thyroid gland (goiter) If you are affected by Hashimoto's disease, your neck may often look big and thick as shown in the Figure. This is because your thyroid gland is swollen and enlarged. This swelling (goiter) often leads to the diagnosis of Hashimoto's disease. The size of the goiter varies widely.