NFL Draft grades aren't the most exciting or useful thing in the world, but what they do is portray a picture of what a person thinks of the players drafted by each team. This is usually based off of the work done on the players beforehand, so it can be helpful.
I find that these draft grades are most helpful, however, when looking at several of them rather than just one or two. Of course draft grades are subjective and done before a single game is played, but when comparing what several different analysts say, an overall opinion begins to emerge.