I have a 5.00 federal reserve note. The date is 1934 C. what does the c stand for?

The letter after the date is a "series" indicator. For many years the practice was to change the date only when there is a redesign, but add a letter suffix when the same design was used but with a different U.S. Treasurer or Secretary of the Treasury. Thus the date doesn't refer to the year of issue like it does with coins, but only to the design. All bills with that design form the year series, and those with a specific letter form a sub-series.

Perhaps the most extreme example of this practice was the 1935 series of $1 bills. They were printed without a design change from that year up till the late 1950s, with letters ranging from A to H.

In recent years the BEP has started to issue a new date series not only when there is a design change but also whenever a new Secretary of the Treasury is appointed. Series letters are reserved for changes in the U.S. Treasurer, so you rarely see any letters beyond A or B. Look at some recent $1 bills to see the pattern.