What do these math equations mean?

I am looking over a script my friend made that shoots out a lightning bolt to wherever your mouse cursor is. Most of it makes sense, however some of the math I can't quite comprehend. Script --I'm only including the function that matters, I will put comments next to math variables I do or don't understand. Also correct me if some of my comments are inaccurate.

function Lightning(pos,pos2,radius,numParts,model)  --Example Arguments: Lightning(plr.Character["LeftHand"].CFrame.p, mousehit.p, 0.15, 6, workspace)
    radius = radius or 0.2 
    numParts = numParts or 10
    model = model or workspace
    local lv = CFrame.new(pos,pos2).lookVector --Positions the bolt in your general vicinity
    local dist = (pos-pos2).magnitude -- Creates a straight line (bolt) between mouse position and you
    local dbp = dist/numParts --Divides the bolt into segments(numParts)
    local last = pos --I think this does something later
    for i = 1,numParts do
        local p = Instance.new("Part",model)
        p.FormFactor = "Symmetric"
        p.Size = Vector3.new(1,1,1)
        p.CanCollide = false
        p.Material = "Neon"
        p.Transparency = 0.5
        p.Anchored = true
        p.BrickColor = BrickColor.new("Bright red")
        local x = math.random(-100,100)/100*dbp/2 --?????
        local y = math.random(-100,100)/100*dbp/2 --?????
        local p2 = CFrame.new(pos+lv*(i*dbp),pos2+lv)*CFrame.new(x,y,0) --?????
        local dist2 = (p2.p-last).magnitude
        local mid = (p2.p+last)/2 
        local m = Instance.new("BlockMesh",p) --Why add a mesh??
        m.Scale = Vector3.new(radius,radius,dist2) 
        p.CFrame = CFrame.new(mid,p2.p) 
        last = p2.p
        game:GetService("Debris"):AddItem(p,math.random(40,100)/1000)
    end
end

The ones with the ????? are the primary cause of confusion, everything else is just minor.