Bollard covers
Bollard covers can be made out of various materials, but the most common one is probably molded plastic.
These bollard covers slip over the bollard post, which is usually steel for this kind of application, and then clip or fasten in place.
A bollard cover serves several purposes and functions, including protecting the bollard from scratches and damage, and when it's in a bright color like this safety yellow version, it also improves the visibility of the bollard, which means it's less likely to be struck by vehicles.
Bollard covers also come in other styles and designs, including elegant versions that are designed more for aesthetic appeal than safety and visbility! They're also available in a variety of sizes to fit over several types of bollard posts.