From what I know of the publishing world, they are probably referring to a "Passive Voice", when instead of saying something forceful like, "Waves crashed upon the deck, making the boat roll unsteadily under his feet." you would instead write, "The waves rose high, rolling onto the deck. The water flowed away, but he still slipped a bit as he walked."
The difference is in how strong and forceful the wording is. Also, any non definates in writing tend to make it sound weak as well, resulting in writing that sounds as though it might be pandering to someone.
I couldn't tell without reading your novel if either of these two apply to your work, but these are the most common answers for the advice "It's not strong enough".
The other response of "It's not what we're looking for right now" is actually a much better one. I've heard that lately Science Fiction/Fantasy has not been selling well, making the publishers cut back on book purchases. They're only picking up ones that will obviously sell well. Any doubt, and they're sending them back.
This is good, because it means that in a more lucurative time, they would probably consider purchasing your novel. This does give you some time to continue revising it as well, continuing to make it better!
If you really want to send it out and get it accepted, you can try to hire an agent to send out requests on your behalf or if you can't get an agent (since many first time writers can't), you may wish to hire an editor on a one time basis.
A professional editor would probably be able to help you figure out exactly what might be making the publishers turn away from it at this time.
If you wanted to post a small scene, just a page or two, then perhaps we would be able to help you figure out how to improve it.
I wouldn't worry too much about the publishers though... Writer didn't come up the the name "the 10-Year Overnight Success" because they weren't getting so many refusals.
Can't live with 'em, Can't feed them to the axe weilding Smiley
The most beautiful thing we can experience is the mysterious. It is the sower of all true art and all science. He to whom this emotion is a stranger, who can no longer pause to wonder and stand rapt in awe, is as good as dead; his eyes are closed.