'Or' is what is called a coordinating conjunction and is part of a series that we remember as:
mais, où, et, donc, or, ni, car
which sounds funny in French, "mais où est donc Ornicar?".
Or has several meanings in English, one of which is yet. It conveys an opposition to what has just been said:
La Météo annonçait du bon temps, or il a plu.
The weather forecast said it would be fine, yet it rained.
Take a look at the following Kwiziq exercise which uses all these conjunctions:
Hope this helps!