The tex2D function usually returns a float in the range of
Think of tex2D as a function that returns a color.
Color components (like red, for example) range from
1.0 where the former is the total absence of that color and the latter is full intensity.
However, as Nathan Reed and other members have pointed out, texture formats that are not normalized may return values outside this range. Here is a StackOverflow post that explains floating-point textures: What exactly is a floating point texture?