When you have two hard surfaces that you need to seal (metal and metal, glass and metal, ceramic and glass, whatever) one of the ways you can seal it is with a ring of soft material that squeezes between the hard surfaces to fill in any gaps between the hard materials and create a good seal.
Best example that I can think of that you might have in your house would be the metal lid to a glass jar in your fridge. Take the lid off the jar and feel the inside of the lid, there should be a sort of rubbery surface.
That isn't really an o-ring as they are usually designed as separate pieces that can be replaced, but it should give you a rough idea.
Since the o-ring failed, a small trickle of liquid fuel trailed down to the engines below, once the fuel was lit it followed the trail back to the initial leak, then tragedy.
And weather conditions was one of the causes, although I think more of a blame would be poor engineering.
That help?