Multiple Barriers with one resistance effect each. It's a thin, colorless, invisible coating covering the body. It was a Multilayer Barrier, consisting of several layers but still so thin that Magic Sense skill could only barely spot it. It didn't require much magic energy to keep going, either—once it was summoned, it barely consumed any at all.
Rimuru acquired this skill when it turns out, he couldn't create a single Barrier with lots of different effects applied to it, but he was free to create multiple Barriers with one resistance effect each. So Ranged Barrier is now linked with his various resistance to create Multilayer Barrier.