In a general sense, yes, but this is not strictly how 'event' or 'return period' is defined. You can have storms larger than a 100 year event, say a 500-year storm, and this could be your largest storm in the past 100 years. Or 100 years can pass without a 100 year event occuring. You probably need several hundred years of record to know with confidence what is truly the 100 year event rainfall. The determination and definitions of "x year events" involve application of a lot of probability and statistics.
You can find a better/more detailed description in most any hydrology textbook. Hope this helps!
CarlB is right in that there are a lot of statistics involved in determining the X-year rainfall event. Although the largest rainfall event in the past may have a bearing on the 100-yr event, there is no real correlation, partly because there are few, if any, events that are exactly 24 hours in length, and there are a lot of stations with fewer than 100 years of record. If you use NWS products (i.e. TP-40, Hydro-35), you'll see that the majority of stations used have far fewer than 100 years of record. (as a side note, NWS will be releasing a new TP-40 in the near future)
One thing to keep in mind is that a 100-yr rainfall event does not automatically mean you will have a 100-yr flood, so be careful in how you make application of the rainfall data.
re: "Is this the highest amount of rainfall for a 24 hour period that was recorded for the last 100 years?"
No.
It is the amount of precipitation associated with a 1% probability of occurrence for any give year. There may be a thousand years without it ever occurring. It may occur three years in a row.
The relationship between the 100 year rainfall and the 100 year flood is much more complex and would require a small book to discuss meaningfully.
