Quick answer
Blender VFX is used to track footage, place 3D objects into shots, composite render layers and finish visual effects work inside a free 3D application.
Camera tracking
Camera tracking estimates the movement of a real camera so 3D elements can match footage more accurately.
Compositing
Compositing combines render layers, footage, masks, color adjustments and effects into a final shot.
Post-production
A VFX workflow often connects tracking, rendering, masking and video output so the final shot feels integrated.