IK Setup Mode
IK setup mode is used to create IK handles and controls for a humanoid character. Reverse Foot (a type of IK) is required for the Automatic Foot Lock feature (link) of Video Capture.
Weight painting is a method for defining bone influences and deformations on a mesh in an intuitive way that functions like a paint brush.
It is primarily used for rigging meshes, where the bone influences (weights) are used to define how bone movements deform vertices on the mesh.
Every 3D character that has bone animation has been weight painted at some point.