Small amount normal. There is normally a tiny amount of fluid there but if it increases it can compress the heart and cause problems. Finding the reason for the fluid such as bacterial infection, tb, tumor, or certain inflammatory disease is necessary to determine the treatment and the prognosis.
Lubricant. In health, there is scanty lubricating fluid between the 2 layers (visceral and parietal) of the pericardium. When the pericardium becomes inflamed (pericarditis), more fluid is made (think of the weeping that occurs in a burn) and the pericardium may fill witih fluid (pericardial effusion) and eventually interfere with cardiac filling (cardiac tamponade) which is a life threatening emergency.