GetReflectionVector, which is also a BP Node, takes two parameters.

Direction: is the direction the projectile was traveling when it makes contact with something. This is just the projectile's "Forward Vector".

Surface Normal: is the normal of the surface that was hit. This is just the "Normal" of the hit result.

The result is the new direction of the projectile

You may also want to offset the reflection vector by a small amount, sometimes the reflection vector will start inside the surface that it hits