Things that are strung are connected together on a string, thread, or wire. A beaded necklace, a popcorn garland, and twinkly lights on a tree are examples of things that are strung.
Things that are strung are threaded or hung along a line. Think of festive lights draped across a patio, or holiday decorations hanging from a string. In a different sense, someone can feel strung, meaning they are tense or stressed, much like a string that is pulled just a bit too tight for comfort and can easily snap or break.