What were the responsibilities of the nobles during the High Middle Ages?
I will answer the question with reference to feudalism as we understand it England, but its form was widespread throughout Europe in the Middle Ages (ca. 1100-1500), with the High Middle Ages in the 13th through the 14th centuries. In England, feudalism during the Middle Ages attains its highest expression after William the Conqueror of Normandy defeated the Anglo-Saxons under Harold in 1066 (after the death of Edward the Confessor, who appointed Harold as his successor). The feudal society in England created by William can be thought of as a ladder, with William on the top rung, and lesser nobles to peasants on the descending rungs.
Because the world of the Middle Ages required almost constant war to a greater or lesser degree, the primary duty of each man was, in its simplest form, to provide political and military support to the man above him on the feudal ladder. And the reason for this "ladder of duty" is that the man at the top rung gave lesser nobles their lands, which included people at the lower levels, and the power that came with land ownership. The next noble, in turn, often gave a portion of his land and its wealth to a lesser noble, and the chain of giving went downwards until it reached the peasant class. Each person on that ladder owed military service and political support to the man above him, and in the best circumstances, then, there was an unbroken chain of loyalty--feudal loyalty--from bottom to top.
The feudal system, on the whole, was designed to produce a guaranteed level of support from one nobleman to the next, ascending all the way to the King, and the system was based on mutual dependence--that is, land, power, and money descended from the king to the lowest-level nobleman and created a solid bond of duty from the lowest to the highest.
If, however, there was a break in that chain or ladder of feudal loyalty, then the nobles often broke apart, made individual alliances with each other, either supported or opposed the king, and the result was civil war.