really dumb noob question.

I suspect you'e talking about the Tamiya product of the same name.

It's essentially Tamiya's version of Mr Surfacer, but only comes in one grade (but can be had in grey or white). It's a sandable, quick drying lacquer based primer/filler. Tamiya's LSP is fully soluble in alcohol so it cleans up more easily tham Mr Surfacer. Apart from that, they are nearly identical.

Ah, I see I see... I meant more along the lines of... what is it used for?

It's essentially a brush-on primer, but can be thinned for airbrushing.

Its main purpose, like any other primer, is to prep a surface for paint. It is used to fill things like sanding marks, scratches etc. Brushed on thinly, it will self-level and the brush streaks disappear.

For larger defects, eg. filling ejector pin marks, it can be applied using a toothpick and sanded smooth when dry. It does shrink as it dries so it may require several applications. I often fill pin marks using this stuff. Apply a drop of LSP in the recessed pin mark using a toothpick and run the tip around the edge of the pin mark to ensure it's filled to the edge. Overfill it so that it forms a raised dome over the pin mark. When fully dry (if you use it this way it may take several hours to harden), use a chisel-edge knife and shave it off flush with the surrounding surface. A few swipes with a piece of wet n dry and you have a perfectly filled pin mark. 

It can also be brushed on in multiple layers to re-contour mis-shapen parts. It sands easily and feathers very nicely. Brush on, allow to dry, repeat, until the part is shaped as required.

If you fill using a regular putty, you can often see this under a coat of paint because the putty is more porous than the surrounding styrene. You can use LSP to "spot prime" over the putty to reduce this effect.

 

is this better, or worse to use to fix small seams and scratches then lets say, squaron green putter?

For certain things, shallow seams, sanding scratches and similar, it's easier to use than Squadron green. You just paint it on with a brush. If using Squadron (or similar) putty, you'd have to thin it first. (I still do this sometimes)

I do use the LSP in ways it wasn't intended to be used - as mentioned earlier, I sometimes layer it and use it as a general filler rather than a primer.

Whether it's "better" really depends on what you're trying to do with it.

For example, imagine you're building a 1/72 F-4 Phantom, where the upper wing parts butt against the fuselage sides, with a hard angle at the wing root. The wing fits closely but there's still a slight, but noticeable seam. This would be pretty nasty to fill with a conventional putty and sand smooth. Using the LSP, you can paint it into the seam, allow it to dry and pretty much just wipe off the excess with a rag/tissue/cotton bud (aka Q-tip) moistened with alcohol and end up with a perfectly filled, clean seam.

Wow thanks, one of my biggest problems is seams that are really small and barley noticable untill the light hits them.  You know the, the kind that putty doesnt like to fill and stick out like a sore thumb

FWIW, I've found that the white liquid typing correction fluid works very well for small cracks and the little sunken areas you often find on models.  Just shake the applicator, apply and when dry make a pass or two with 400 or 600 grit sandpaper (usually takes less than five minutes to do the entire job). From what I've seen, it's compatible with most modeling paints (I use enamel paints almost exclusively).  It's cheap and easy to find and use (Pental is the brand I use and I got my last tube @ Wally World).  Just a suggestion

If you are using Squadron, I took the advice of another and spread it with an alcohol-impregnated finger and it makes ALLLLL the difference!!