Hitting your mark simply means arriving on time and exactly at the marking on the floor where you need to stand at a particular point in your scene. Obviously, you want be able to do this walking tall, rather than appearing as if you are searching for a lost contact lense or something.
The easiest way to hit your mark is to notice what is easily at eye level where your mark is, and memorize this location. For example, if your mark is where the corner of a piano is in the scene, or whatever.
Once you know these locations without having to change where your eyes are looking, practice moving fluidly (or rather, in character) to these locations, and coming to natural stops. If you encounter any awkwardness with this, you may want to make a note of it to the director, who might decide to either move your mark, or give you some coaching for how they want you to approach your mark.