A "bard" isn't just any musician. They're a highly educated experienced user of language. Telling stories, composing poems & songs, and being in the employ of a noble to do so. In popular culture there's also often the implication that they use these skills and that access to be involved in espionage in some way.

I think Robert Jordan's Wheel of Time series does an especially good job of this. It has gleemen, which are reasonably well-trained in music, storytelling, and other performing arts. Gleemen travel around from town to town making their living playing at taverns and the like. Then a step up from gleemen it has bards, which are more well trained and who perform explicitly for nobles. In either case you can expect a great level of artistic skill, but I'd be shocked to hear of an illiterate bard, but maybe only mildly surprised to hear about an illiterate gleeman.