The Material Sampler samples an Input Shape and then outputs values of between 0-1 depending on the value (lightness/darkness) of the shape (e.g. black = 0, white = 0). Alternatively it will output the color if connected to an attribute that supports it (e.g. Fill Color).
Value - a multiplier.
Input Path - connect a Shape with a Material to sample from.
Graph - use the graph to remap the output values.
Offset - add/subtract a value to the output(s).