Can someone give me an explanation of how this LocalScript works?

I was trying to make my own flying script, but I failed in the end. I went to the toolbox and got a LocalScript. I get everything except the Fly function, since I've never worked with BudyGyro and BodyVelocity. I would appreciate if anyone could explain how the Fly function works. Thx in advance!

repeat wait() 
until game.Players.LocalPlayer and game.Players.LocalPlayer.Character and game.Players.LocalPlayer.Character:findFirstChild("Head") and game.Players.LocalPlayer.Character:findFirstChild("Humanoid") 
local mouse = game.Players.LocalPlayer:GetMouse()
repeat wait() until mouse
local plr = game.Players.LocalPlayer 
local torso = plr.Character.Head 
local flying = false
local deb = true 
local ctrl = {f = 0, b = 0, l = 0, r = 0}
local lastctrl = {f = 0, b = 0, l = 0, r = 0} 
local maxspeed = 400 
local speed = 5000 

function Fly() 
    local bg = Instance.new("BodyGyro", torso)
    bg.P = 9e4 
    bg.maxTorque = Vector3.new(9e9, 9e9, 9e9) 
    bg.cframe = torso.CFrame 
    local bv = Instance.new("BodyVelocity", torso)
    bv.velocity = Vector3.new(0,0.1,0) 
    bv.maxForce = Vector3.new(9e9, 9e9, 9e9) 
    repeat wait()
        plr.Character.Humanoid.PlatformStand = true 
        if ctrl.l + ctrl.r ~= 0 or ctrl.f + ctrl.b ~= 0 then
            speed = speed+.5+(speed/maxspeed) 
            if speed > maxspeed then 
                speed = maxspeed 
            end 
        elseif not (ctrl.l + ctrl.r ~= 0 or ctrl.f + ctrl.b ~= 0) and speed ~= 0 then 
            speed = speed-1 
            if speed < 0 then 
                speed = 0 
            end 
        end 
        if (ctrl.l + ctrl.r) ~= 0 or (ctrl.f + ctrl.b) ~= 0 then
            bv.velocity = ((game.Workspace.CurrentCamera.CoordinateFrame.lookVector * (ctrl.f+ctrl.b)) + ((game.Workspace.CurrentCamera.CoordinateFrame * CFrame.new(ctrl.l+ctrl.r,(ctrl.f+ctrl.b)*.2,0).p) - game.Workspace.CurrentCamera.CoordinateFrame.p))*speed 
            lastctrl = {f = ctrl.f, b = ctrl.b, l = ctrl.l, r = ctrl.r} 
        elseif (ctrl.l + ctrl.r) == 0 and (ctrl.f + ctrl.b) == 0 and speed ~= 0 then 
            bv.velocity = ((game.Workspace.CurrentCamera.CoordinateFrame.lookVector * (lastctrl.f+lastctrl.b)) + ((game.Workspace.CurrentCamera.CoordinateFrame * CFrame.new(lastctrl.l+lastctrl.r,(lastctrl.f+lastctrl.b)*.2,0).p) - game.Workspace.CurrentCamera.CoordinateFrame.p))*speed 
        else 
            bv.velocity = Vector3.new(0,0.1,0) 
        end 
        bg.cframe = game.Workspace.CurrentCamera.CoordinateFrame * CFrame.Angles(-math.rad((ctrl.f+ctrl.b)*50*speed/maxspeed),0,0) 
    until not flying 
    ctrl = {f = 0, b = 0, l = 0, r = 0} 
    lastctrl = {f = 0, b = 0, l = 0, r = 0} 
    speed = 0 
    bg:Destroy() 
    bv:Destroy() 
    plr.Character.Humanoid.PlatformStand = false
end 

mouse.KeyDown:connect(function(key) 
    if key:lower() == "e" then 
        if flying then
            flying = false
        elseif plr.Mode.Value == 'Flying' then --I added "plr.Mode.Value == 'Flying' "
            flying = true
            Fly() 
        end 
    elseif key:lower() == "w" then 
        ctrl.f = 1 
    elseif key:lower() == "s" then 
        ctrl.b = -1 
    elseif key:lower() == "a" then 
        ctrl.l = -1 
    elseif key:lower() == "d" then 
        ctrl.r = 1 
    end 
end) 

mouse.KeyUp:connect(function(key) 
    if key:lower() == "w" then 
        ctrl.f = 0 
    elseif key:lower() == "s" then 
        ctrl.b = 0 
    elseif key:lower() == "a" then 
        ctrl.l = 0 
    elseif key:lower() == "d" then 
        ctrl.r = 0 
    end 
end)

Fly()