Two of the most common mistakes beginners find is either moving your welding rod too quickly across the seam or too slowly. I'll demonstrate both.
Here you can see two good welds flanking a weld that was done too quickly. A good weld and good speed is indicated by an even spacing between these arced ridges. The weld is a consistent height showing that you had good penetration and good deposit. As the welder sped up you can see that the distance between those ridges increases and becomes erratic. The weld is quite high and inconsistent demonstrating poor penetration and ultimately a poor connection between the two pieces.
Conversely, if you let the welding rod stay in one place too long you risk destroying or blowing out the base metal itself. I'm going to deliberately do this so you can see what might happen. Here you can clearly see what kind of damage that can be done if the welding rod is allowed to stay in one place for too long.
Another common problem besides this lateral speed of the welding rod is keeping a consistent distance between the tip of the rod and the work itself. Remember, your rod is being consumed, so it's growing shorter all the time. As you can see I deliberately let the rod be consumed, and therefore the arc length got longer and longer until it finally failed.