A: Closed reduction is where the bone has broken and not punctured through the skin.
If your bone has stayed in place and is in good alignment it may be possible to apply a cast without the need to manipulate the bone.
If however the bone fragments have moved and are no longer in proper alignment, your orthopedist may be required to perform a 'closed reduction'. A "reduction" simply means the bones are re-aligned.
How is that done?
By manipulating it. By physically moving, twisting, pulling and pushing into the correct position. If the fracture has not punctured the skin, it may be possible to manipultate the bone without the need to open you surgically.
There are some cases where the fracture requires surgery to fix the broken bones. That is known as an "open reduction."
If surgical hardware is needed to hold the bone fragments together, that is often known as "internal fixation."