The seatpost of your bike is inserted into the frame of your bike at the top and holds the saddle. When the seatpost is stuck you cannot adjust the height of the saddle, a crucial and necessary adjustment for cycling power and efficiency. Seatposts can become stuck in any frame material, including frames made of carbon fibre. This can be a frustrating issue to deal with, but there are some ways to tackle the problem.
Completely loosen the seatpost bolt at the top of the frame, where the seatpost enters the frame. Wrench sizes vary, and this is often an allen-style bolt, so Allen wrenches may be necessary.
Straddle the bike top tube, facing the seat, and twist the saddle side to side. If the saddle moves, attempt to gradually work the tube up and down to break the grip. If this isn't successful, go to Step 3.
Spray or drip a light lubricant on the junction of the seatpost and the top of the frame. Give the lubricant some time to work down into the frame (abut an hour) then attempt to move the post. If this is unsuccessful, go to Step 4.
Drip penetrating oil into the frame at the junction of the seatpost and frame. Let the bike stand overnight. Stand over the frame and tap firmly with a sideways stroke on the nose of the saddle, using a rubber mallet to break the grip.
If you have not loosened the seatpost after this attempt, take the bike to a bike shop with a mechanic. Applying more force than you have to this point may damage the carbon frame, so the work should be left to a professional if you have been unable to move it.