In What are the jointer-specific steps for removing warp in lumber? a planer sled is suggested for milling twisted lumber rather than a jointer. What is a planer sled, and how is it used for removing twist?
A planer sled is a flat, rigid box or piece of material that supports your workpiece and holds it in a specific orientation as it passes through a thickness planer.
By holding a twisted or otherwise warped piece of lumber in a fixed orientation as it passes through the planer, the sled allows the planer to chop off the high spots, effectively jointing the top surface flat. Afterward, you can flip over the workpiece and send it through the planer again (with or without a sled) for one or more passes until the other side is flat and parallel to the first (flat) surface.
Most designs use shims, wedges, or screw levelers to fix the piece in one orientation and prevent it from rocking or from being bent flat (e.g., if the board is cupped or bowed) by the planer's pressure rollers. Most designs also use hot glue, screws, and/or a stop at the leading edge at the front of the sled to secure the workpiece in place and keep it from sliding off the sled.
Here are a few examples...
The simplest designs often involve a thick slab of MDF or particle board, with hot glue and business card or playing card shims to secure the workpiece in place:
Alternative, lighter-weight designs involve attaching rails to the sides of a workpiece (especially useful with longer, thicker material):
There are also many other designs with various usability features such as built-in levelers and adjustments...
Based on plans in ShopNotes issue #138:
Based on plans from Fine Woodworking:
To add to Rob's answer: A planer mills the wood to a fixed height above the bed. If the piece is warped or twisted, the warp will still be there upon exiting the planer, and at best you'll have a warped piece with constant thickness. (Not even that, if the piece rocks differently each time through the planer.)
With the sled or rails holding the wood in a fixed orientation, the planer instead acts like a wide jointer, cutting a flat face. This does allow the thickness to vary, but after one face is flat you can flip the wood over, use that flat face as a stable reference, and plane the other face both flat and parallel.
Think of it as a home brew alternative to buying one of the fancy jointer/planer combo machines, if you already have a small jointer (or are willing to joint narrow pieces by hand).