It's a Valyrian steel dagger. If Arya gets her hands on the thing, it means she'll be equipped to take out white walkers.
I wonder how she gets her hands on it though. Wasn't Catelyn the last one who had it? She died at the Twins, but clearly, Arya didn't find it there. I was thinking maybe Cat had left it at the Eyrie after Tyrion's "trial", but it feels like a slight detour from Arya's current path towards King's Landing. Oh well.
To be honest, I can't see the dagger reveal all that much about Peter Baelish. Bran however, is going to arrive at Winterfell any time now, and I think that's when the Starks will learn a bit more about Baelish.