Inserts cradle the bottles inside the box or crate. They come in either wood or cardboard. The wooden inserts slide into the inside of the crate. Below is a picture of wooden inserts for a wine crate:
The above picture is the inside of a 6 bottle crate with two inserts. The top and bottom inserts slide into the crate because the crate has routed grooves as shoown above. The 4 routed grooves allow the inserts to slide in and out.
When adding 6 bottles to the above crate, you would remove both inserts and place three bottles at the base/bottom of the crate. You would then place the inserts over the three bottles, and then place the remaining three bottles on top of the inserts. All will fir perfectly, and wont move in transit.
Cardboard inserts are a little different, and they're generally used for crates without these routed grooves. The cardboard inserts are crafted in the shape of a wine bottle. Below is a picture of two 6 bottle wine crates with cardboard inserts:
As you may see, there are two different sizes on the cardboard inserts. This is because the left hand crate is from Bordeaux, and the right hand crate is from Burgundy. The wineries in Bordeaux and in Burgundy often make differently sized bottles.
This is a flat crate that stores the bottles across from left to right. They aren't stacked like the other ones above.
The inserts in the Winter crate are in 4 different pieces: The top 2 inserts (where the lid is) are both removable as is the bottom 2 inserts (just above the winer logo) and they're both removable as well. There are a total of 4 pieces here, not two. It looks like the inserts are each one piece but they're not.
How you'd add the bottles: Remove the top parts of both inserts and place the bottles on the bottom insert. You would then put the top inserts back above the bottles to secure them in place.
Now the bottles are fully protected in the crate and won't move in transit. Close the lid and they're good to go!