When you skip a question on the SSAT, you neither gain points nor lose points.
If you answer a question correctly, then you gain points; if you answer it incorrectly, then you lose one quarter of the points you would have gained for answering the question correctly. But, again, if you leave the question blank, you don’t gain anything or lose anything.
For this reason, most test-prep courses and tutors advise you to guess on questions you can’t answer. In my opinion, this guessing advice is a really, really bad idea–for more on this topic, please check out my post on guessing on standardized tests.