His lore pegs him as a celestial vagabond, showing up when cosmic equilibrium is under threat and attempting to steer Runeterra (that's LoL-land) away from catastrophe. I'm now idly wondering whether the lore here is a hint at something important.
The basic logic behind the games in League of Legends used to be that these magical battles take place as a kind of political conflict resolution scheme. In September 2014, Riot announced they were giving League's lore a do-over and clarified that "Summoners, the Fields of Justice and the Institute are not canon within the fictional world of Runeterra". The idea was phrased thus:
"From champion interactions to bios to events (and beyond), we aim to expand the scope of League’s story and pursue a more dynamic and wide-ranging world fit for the outsized capabilities and personalities of our champions."
That's why I'm wondering whether the description of Bard as only appearing when Runeterra is under threat is part of a wider story or will prompt a particular event or narrative. Maybe called RuneTERROR. Who knows?
Anyway, let's look at what Bard does. He seems to have a little minigame component to his passive ability where you have to collect chimes from the map. They give you brief boosts for speed, experience and mana as well as empowering little spirits that Bard attracts and that he can use as projectiles. This might be something useful for newcomers to the support role as it should incentivise moving around the map. It might also incentivise risky exploration and abandoning your carry, though.
Bard's Q ability is a skillshot that slows or stuns depending on the circumstances, his W is a kind of healing mine thing that will help with sustainability when Bard wanders off, his E lets you go on a one-way journey through a usually-impenetrable wall and his ultimate ability puts units in a targeted area into stasis, making them damage-immune for the duration.
From the video it looks like you can apply the ultimate to the dragon so that could lead to some really cool saves and steals. In pub games I suspect there will be at least a week when the champion first arrives where games are awash with the question "WHY DID YOU FREEZE ME YOU JERK?" That's certainly been a frequent question since Winter Wyvern arrived in Dota 2.
Riot say of Bard's addition to the roster,
"We've looked at the idea of roaming supports before. Alistar was one, for a while, before he roamed so well you guys transitioned him into a death-dealing jungler thanks to his nigh on unstoppable W+Q combo and extreme tower diving capabilities. So we made changes, nerfed his extra monster damage, and put him back in his traditional support pants. But we still liked the idea, and so started thinking of ways that we could bring a new champion into League that had all the hallmarks of a roaming utility support."