It seems to have multiple meanings and connotations all blurring into each other. Possible meaning include:

• "Treating someone as a means rather than an end." I'm generally OK with treating people as means, as are most LWers AFAICT, but relationships (and to a lesser extent morality) is expected to include having their desires as part of your goal structure.
• "Treating someone as not having goals of their own." Objectively wrong, obviously, and if you genuinely believe or alieve this you're likely to run into some problems, I guess.
• "Treating someone as only existing only to serve as a status symbol, "sex object" or housekeeper." More subtle than the second one, as it relates to goals rather than beliefs, but ultimately has the same problems if you're a neurotypical human or similar.
• "Focusing on the utility someone's body provides, rather than their mind/personality." Depends on your goals, I guess, but probably not conductive to healthy relationships and many would argue it causes all sorts of subtle societal problems.

Most people mean many or all of these when they say "objectifying" due to connotations and sloppy terminology. A few also include "Treating someone as governed by instinct rather than as a sentient being", especially when discussing PUA.

Does that answer your question?