Help what is inTangentVector and outTangentVector

[FZMv587]

Hi!
I made rope use scripting, when I set "control point" with this code.

Code:

AddControlPoint (Vector3 position, Vector3 inTangentVector, Vector3 outTangentVector, Vector3 normal, float mass, float rotationalMass, float thickness, int phase, Color color, string name)

I know Tangent Vector but i confused about inTangentVector and outTangentVector, why there is two?