If something's lacking, there's not enough of it. Your new house may be lacking when it comes to comfortable chairs, leading your friends to lounge on the floor instead.
If there's a shortage of something, it's lacking. You might also describe something that's deficient — or not quite good enough — as lacking. If a substitute teacher is lacking in skill when it comes to controlling a rowdy class, he'll be exhausted by the end of his first day on the job. Finally, you can use this word in the sense of "absent": If your older brother's appetite is lacking, he might be coming down with something — usually he can eat an entire pizza on his own.