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.
Swollen neck lymph nodes and Tracheal Stenosis and Abdominal symptoms (2 causes) Swollen neck lymph nodes and Tracheal Stenosis and Acute fever in children (2 causes) Swollen neck lymph nodes and Tracheal Stenosis and Acute Vision Changes in Both Eyes (with pain or inflammation) (2 causes)