Urethane (or more formally polyurethane) is an artificial polymer. As a protective coating on metal baskets, it is durable and very protective under UV rays. It is not great at inhibiting rust, but that is less a factor when coating rust-resistant material such as titanium, stainless steel or aluminum. The average coating thickness for urethane is 0.032” to 0.058”.
One resource on the topic is the Urethane Blog.