A nacelle pylon is a large support beam that connects a warp nacelle to the hull of a starship.
Presumambly, nacelle pylons are constructed of the same material as the rest of the vessel (duranium or tritanium in Federation starships) and would obviously need to be strengthened for inertial purposes, since the pylons would clearly be a weak point in the hull while the vessel is at warp speed. The pylons also provide personnel access to the nacelles via the use of Jeffries tubes, and also bear the power transfer conduits from the warp core.
On Federation vessels, nacelle pylons are often fitted with explosive bolts, so that the nacelles can be jettisoned in an emergency.
The designers of the USS Defiant eliminated the need for nacelle pylons by having the nacelles tucked against the ship's hull, giving a smaller target silhouette.
On Intrepid class vessels, the impulse engines are built into both nacelle pylons, possibly so that they would be clear of the warp nacelles.