If an object is riddled with something, that means it's covered by that thing: for example, a tree trunk might be riddled with tiny holes made by the beaks of woodpeckers.
The adjective riddled most often describes something damaged by or spread full of holes, like a rusty old mailbox riddled with bullet holes or a block of Swiss cheese riddled with holes. You can also use it figuratively: "It's so sad, her dog is riddled with cancer," or "The plot of your novel is riddled with holes — it just doesn't hold up." Riddled comes from the Old English hriddel, "sieve."