your feed store or petstore should have it (just ask for electrolytes) or in a pinch you can use diluted pedialyte (a childrens electrolyte) or if nothing else you could use gatorade altho these last two are generally short term emergency measures.
re your question as to "what" are electrolytes... some excerpts from a couple articles :
Excerpts which may answer your question below:
http://www.thepoultrysite.com/articles/1209/acidbase-balance
"Electrolytes are compounds capable of dissolving and dissociating into positively and negatively charged ions. These ions are the means by which the body maintains normal acid-base balance. There are many environmental, and nutritional factors that can challenge the acid-base balance of an animal. However, the animals body has many and varied, homeostatic mechanisms which it calls on when necessary, to maintain an acid-base balance environment conducive to good health and optimum performance. ....
....Metabolic Acidosis
Definition
Metabolic acidosis occurs with an increased production or ingestion of organic acids, or excessive loss of HCO-3. Reduced efficiency of the kidneys to excrete H+ or to re-absorb HCO-3 can also result in acidosis.
Conditions
Several common metabolic conditions can lead to metabolic acidosis, one of these is starvation. This is often referred to as ketosis and results from the incomplete metabolism of fatty acids that are mobilized by the body because of the lack of available glucose as an immediate source of energy for the cells. Another is lactic acidosis, caused by low oxygen uptake by the lungs or excessive oxygen demands by the body (eg. fever, seizures). Decreased hepatic blood flow, as experienced with cold exposure, can also result in decreased lactic acid metabolism and thus lactic acidosis. Diarrhea can result in HCO-3 loss and thus trigger metabolic acidosis. Diuretics that act as carbonic anhydrase inhibitors and ingestion of acidifying salts like ammonium chloride, can also cause the condition. ....."
http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/206918.htm
MERCK Veterinary Manual
"......
Electrolyte Imbalance:
Most commonly, electrolyte balance is described by the simple formula of Na++ K+ - Cl- expressed as mEq/kg (or mEq/g) of diet. Generally, an overall diet balance of 250 mEq/kg is optimal for normal physiologic function.
The primary role of electrolytes is to maintain body water and ionic balance. Thus, requirements for elements such as sodium, potassium, and chlorine cannot be considered individually, as it is the overall balance that is important. Electrolyte balance is affected by 3 factors, namely the balance and proportion of these electrolytes in the diet, endogenous acid production, and the rate of renal clearance.
In most situations, the body attempts to maintain the balance between cations and anions in the body such that physiologic pH is maintained. If conditions in the body result in a shift toward acid or base conditions, physiologic defense mechanisms alter metabolism to maintain normal pH. .....
Electrolyte balance can therefore more correctly be described as the mechanisms that must occur in the body to achieve normal physiologic pH. Electrolyte imbalance causes a number of metabolic disorders in birds, most notably tibial dyschondroplasia and respiratory alkalosis in layers. ..... "
Hens go broody when you don’t want them to… and won’t go broody when you do. 
