I was reading how to plant catnip seed, and noticed that it said 1-2 weeks before the last frost. Does that mean to plant it when the ground is still frozen?
No, it does not mean when the ground is frozen; it means before your last frost date. Frost and frozen ground are not quite the same thing. This answer here, though it's actually about when to cut grass, has information regarding the difference and may be of relevance How long after last frost before lawns need mowing?.
Usually, two weeks before your last frost date will mean the ground is not frozen solid, as it might have been in cold regions during winter; the height of the sun in the sky and longer days should ensure the ground is no longer frozen, but it's still entirely possible for a frost to occur overnight without the soil actually freezing. If you've had an unusually cold and long winter, the scheduled last frost date might take place later, so conditions where you are should be taken into account. You can check if the ground is still frozen by trying to stick a garden fork into it - if it goes in fairly easily all the way down, it's not frozen.
No, it doesn't. If the ground is frozen, you're a long way until the last frost.
Frost is an icy substance that forms on the ground, plants, car windshields, and things. It tends to form in the early morning.
Since weather people take the temperature higher up, it can be freezing lower down, and frost even at 41° F. (which I have witnessed), although in my experience, frost above 32° F. is much rarer than at or below 32° F., and I've often felt confident planting as long as it's consistently above 32° F. It's still a good to idea to protect some kinds of plants for a bit, though.
Dave's Garden has a handy page for looking up your last frost date.
Since I am actively buying seeds from various sources, I came accross the same directions for planting/sowing as you did, and from many vendors.
My understanding is that 'frost' means 'the lowest air temperature during the day was less than 0° Celsius'. Exact date of 'two weeks before last frost' is, of course, your estimation, based on past experiences, current season, and weather forecast.
Your soil may or may not be frozen at that moment. Mine was not frozen at all during whole winter, but I am zone 8a.
The bottom line, this is just a rough guideline for planting/sowing date, a convenient way of specifying a good planting/sowing date that is not a specific date (like March 12th), but dependant on particular hardiness zone and current weather seasonal condition - and is not directly related to the soil being frozen.