The Particle Constraint modifier uses a floating point value to specify the particle to be constrained to. This matches the rest of the particle system, and allows it to interface with things like the Particle Sample nodal shading node. However, from a user point of view, a floating point index can be quite difficult to use. The Generate ID node solves this by providing a conversion from an integer index to a floating point index, used by the particle system.
Index: The integer index used to generate the floating point ID.
ID: The resulting floating point particle ID.