You can choose here whether the computation will be done in the original units of the data, or converted to standard units. For a correlation this makes no difference, but other operations such as the mean or a regression depend on the units used.
The system will try to guess what kind of quantity is represented and determine the standard units accordingly. This is not foolproof but works most of the time.
Some data do not have valid units. Please inform me if you find standard data without units. If you upload your own data, you can indicate the units in an ascii file by a line "# var [units]" in the header. In a netcdf file use the standard "units" attribute, in GrADS I use the convention that the units are in square brackets after the long name.