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The key to answer this question is to know how to derive IS and LM. For IS, the causal relationship is from interest rate to GDP; larger interest rate, less investment, and therefore smaller GDP. IS represents an equilibrium relationship between interest rate and GDP.
Exogenous factors (i.e., other factors besides interest rate and GDP) cause the IS to shift either "right" or "left". It means that, given the interest rate, an exogenous change (say an increase in government spending) may casue IS shift to the right. Therefore, any points on the "right" or "left" of IS represents disequilibrium. On the right (left) of the IS, given the intereest rate, the actual GDP is greater (smaller) than the equilibrium GDP. Therefore, there is an excess supply (demand). Equivalently, it states that whether saving is greater or smaller than investment. Y > C + I + G (a closed economy) means Y - C - G (Saving) > I and vice versa.