First woodworking project here, sanding a table that had previously been stained to get it to be smoother. I worked my way up from 120 to 220 and then lightly sanded with 320. Now there are these funny white spots everywhere that sort of resemble skid marks. The white spots are slightly raised. Any idea what it could be? 
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2I have seen those white dots on framing lumber before. I believe they are a byproduct of the milling machinery, probably a result of the treads gripping the boards. Why they are raised instead of depressions may be one of life's mysteries we may never solve!– Ashlar ♦Commented Mar 4 at 18:02
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2 Answers
I agree with graphus's statement that it needs more sanding to get ride of those marks. I would add, to save you a lot of sanding time. If you have a hand plane such as a smoothing plane, you can remove those marked much faster than sanding.
I'm not sure what the white spots are since you said they're raised, because the first impression was the reverse — that it was white finish left in depressions. But regardless, the wood hasn't been sanded enough. You can also see clear rotary marks from a table saw/circular saw/tracksaw blade, another sign wood hasn't been smoothed off enough1.
When sanding to remove stubborn finish and/or to flatten — not the ideal way to do this — you may need to step down in grits to remove material efficiently. Use 80 at least and even 60 or lower sometimes.
It may seem counterproductive to introduce such coarse sanding scratches but the time saved in the initial phase when removing lots of wood usually more than makes up for the much longer sanding time needed if starting with 120 or thereabouts. This is especially the case if using a typical rotary or random-orbit sander; these were originally intended as finish sanders only, and not for heavy material removal2.
I worked my way up from 120 to 220 and then lightly sanded with 320.
FYI one almost never needs to sand wood to above the 180-240 range3. Any good finishing book will explain why this is the case and should also give pictorial examples, especially in relation to staining.
It should also explain why you generally want to complete sanding by hand-sanding in the direction of the grain if the last power sander to touch the wood was a rotary one.
1 By normal standards, 'rustic' stuff may retain such texture deliberately.
2 Note this does not include some high-end rotary sanders made today.
3 There are exceptions, including if painting (150, even worn 120, can be enough) or when waxing or oil-finishing certain hardwoods, especially denser tropical species.
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1Thank you so much!! This is incredibly helpful. I will try all of this. I also found the fact that the white spots were raised to be strange. I wondered if it could be some accumulation of wood dust while sanding, since the stain is the more red color and the white appeared progressively as I sanded. But either way, it does seem that it needs more sanding. Will try the coarser grit! Commented Mar 3 at 18:12
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You're welcome, glad to try to help. I don't need another chosen Answer so there's no reason to change your selection but just for the future, the Answer that's most helpful would be the one that should get the tick, not one that merely agrees with that other Answer. I didn't mention other (very good, arguably better) options to sanding like hand planing to smooth the wood further only because the only tag selected was sanding, and that was the focus of your Question. Plus I'd already tackled extraneous stuff that wasn't directly what you'd asked, but was related enough to include.– GraphusCommented Mar 4 at 8:43