What does "first true leaves" mean?

I've read that you are to begin feeding nutrients when the plant has it's "first true set of leaves". Does this mean the first 2 jagged leaves (that show up right after the cotyledons) or the first leaves with their own stems?

I'm seeing yellowing edges and brown spots on the leaves of my seedlings that have been growing sooooo slow(about 3 weeks now w/2 sets of single leaves). They are staying super compact and growing these masses of growth on top (looks like 8 more leaves starting to grow all at once in the same spot). I don't know if it's related or genetic or what, but I figured I'd mention.
Also, I may have had my 400w MH too close for a day or two. I've since moved it to about 3-4' away, but it still seems to be getting worse.
I'm using an aero hybrid w/ seedlings in rw, inside of hydroton. Air is about 78'f day, 70' night, water is 70'f, distilled 005ppm. Been running mister at 1hr on / 1hr off. Humidity is also a little high, was up to 80% at one point.
If necessary, I can try to get pix, but I'm hoping maybe i can try to get some help without.

I would look for more advice but here is my 2cts. yes its your first set of jagged leaves. 2nd it sounds like you did right by moving the light up, dont know how close u had it at 1st but at seedlings too close & strong of light could cause damage (heat stress). 3rd i would make sure they are getting enough water, assure your mister is effectively watering your plants and maybe decrease your time interval w/ it as well,

i'm fairly new at growing myslf but have been successful in finding my mistakes eaRLY and made corrections quickly resulting in decent grows. at the very least what I said should only help and do no harm and be sure not to over feed them as well at this point, light, water is all they need mainly & oh yeah your temp may can come down 3-4deg as well. i shoot for 72-75 daytime w/ a drop off to about 65-68 at night.

make sure to watch your humidity, that was way too high, off the top of my head it should be around the 30-50 pct . like i said im no expert but this sholud get you some lifesupport for your babies until more help arrives

So, I should go ahead and start feeding a low dosage of nutes now?

I'm seeing some drooping of the leaves. This would be the humidity, right? I'm working on a drain for the dehumidifier, then I should be able to keep around 35-40%.

Do you think the lack of nutrients may be causing the yellowing or the brown spots?
The light was about 16" at closest, now up to about 3'. (still looking pretty crappy, though).
Growth is very slow, except the growth on the top of the plant, the leaves aren't really getting bigger. I did notice, though that the roots have started protruding from the bottom of the 6" cups, maybe i'm just being a paranoid newb...
I appreciate all of your help.

sound like some heat stress...but I'm leary as you said your plants are 3 weeks old with only one set of tru leaves I call my first set of fans my first tru leaves at 3 weeks you should be farther along pics would help