The stringers hold the edges of the tub up so if someone sets or stands on it, it doesn't break off. If the tub was CI, they could hold the tub up, but it's still best to bed it into something in case the fastners rust or pull out.
Sand mix is a mix of mostly sand and portland cement. You can buy it premixed, or mix it up yourself with around a 4:1 ratio (sand to portland cement). It doesn't pour! You'll need to shovel it. Think wet beach sand. You mix it with enough water (you'll need something like a hoe to mix it) so it just holds together if you squeeze a ball in your hand. To help keep it from drying (it want to cure, not dry), put something on the wood subflooring - roofing felt or plastic, to keep the wood from sucking all of the water out before it can cure.
Stuctolite works as well, as can plaster of paris (but it sets up really quick).
You want it thick enough to hold together - that's nominally at least 1-1/4".