So, you've said it several times, and I presumed someone would have gotten to this by now, but they haven't. Barring that, I would have thought a simple Google search on the word would tell you what you need to know. Again, apparently not. So, here. You're welcome...
Bards are from ancient Celtic history & mythology: Irish, Welsh, Scottish, some from the Gaulish (old France/northwestern Europe). The most famous, generally cited, is a guy named Amergin. Another somewhat famous example, often conflated with stories/legends of the wizard of Arthurian myth, Merlin [I certainly hope you've "heard of him/know where he comes from?"], is a great magician/historian/druid, Taliesin.
They were historians/chroniclers, learned men, knowledgeable in all matters of lore and the laws of the men and lands among whom they lived. They served as diplomats/emissaries, judges, talented orators/speech givers, heralds and advisors of their kings, and initiated magician-priests, and, yes, inspirational warriors and/or battle-leaders/warlords of their peoples. They use wit, knowledge, and magic. Powers -as they usually do in Celtic legend- are closely tied to the natural world -calming seas, creating mists, calling forth the winds or plants to grow, and the like- and a smattering of transformation -altering one's appearance, turning into animals, etc... They are capable warriors, as all able-bodied people among these cultures are, but rely moreso on wit, often with cunning/trickery, and their supernatural abilities born of their abundant knowledge, years of training, and sometimes (as with all myth) some supernatural/semi-divine origin. Playing the harp is fairly ubiquitous (for celtic culture) -as it was a magical skill available only to the most learned and gifted (talented) of men.
A learned wandering guy with musical talent and a little knowledge about just about everything is merged in D&D (since 1e) with the medieval trope of the "wandering minstrel/troubador." So, the "magical musician" bard with a mandolin or lap harp, as a stringed instrument is almost always noted as a medeival instrument/music. This has burgeoned out to include pretty much any and all myth that includes music/instruments of all kinds: reeds/flutes/pipes (a la "Pan Pipes," the Pied Piper), percussion (a la shamanic drum circles), and brass/horns (the trumpets that fell the walls of Jericho from the Bible) trope of D&D was born and has solidified (not necessarily for the better, imho) over the editions. They are further mingled and broadened over editions with Scadanavian legend/culture of individuals known as "skalds."
Google Amergin and Taliesin..They will tell you everything you need to know about what a bard is supposed to be. More broadly, Irish and Welsh mythology/legends/ancient history.