What does it mean to have high red blood cell count and high white blood cell count at the same time?

Possibly serious. Need more details. Could be normal variant or underlying serious bone marrow condition.
Depends. A CBC - complete blood count - has many values. 3 cell types are white blood cells (wbcs), red blood cells (rbc, but measured as hemoglobin - hgb), and platelets (plt). All cell types can be increased when someone is dehydrated. Wbc can increase from infection, inflammation, leukemia, etc. Hgb may be high from low oxygen (smoking, sleep apnea) or from blood disorders (polycythemia vera).