What does it mean to have dual input range?
[…] the Murata NCS12 series [has] input ranges of 9-36V and 18-75V. Now what does that mean?


Remember that this is describing a series of parts, which includes many different parts with different characteristics.

The parts with a nominal 12V input (order codes NCS12x12xxx) have an input range of 9-36V.

The parts with a nominal 48V input (order codes NCS12x48xxx) have an input range of 18-75V.

There is no single part that can accept voltages from both ranges.




  Also what does the 10% load efficiency mean?


I think you're confused. There is no such value in the datasheet.

The datasheet gives a range of efficiencies (e.g, 78-83.5% for the NCS12S1203C), and gives specific details on input currents under 10% and 100% load at specific input voltages, but there is no "10% load efficiency" specified. It's possible to calculate an efficiency at 10% load from the current values in the second table, though.




  So would this be great for my application of inputs 12,24,36,42V 300mA peak 1A ?


No. There is no part in this series which supports that range of input voltages.