A value of type “double ” is a double precision floating point number. For the computer, it is 64 bits where one is used for the sign (+ or -), 11 are used for an exponent (power) of 2, and 52 are used for the fractions…

In a brief: double values can be positive or negative, can represent fractions (e.g. 0.25 or 3.75) - and hence are usually the result of division/multiplication operations. They are being used where the precision of a Single value is not sufficient. If you are interested in how a double is technically encoded see https://en.wikipedia.org/wiki/Double-precision_floating-point_format

The appropriate casting function is Math.Cast.ToDouble.

An example is

<step pause="0.001">ActionScript.SetProperty.Double('Editor.Shapes.PhaseOffset', 180)</step>

Here, the property Editor.Shapes.PhaseOffset expects a Double and is set using ActionScript.SetProperty.Double to the value 180.