I found a script but when I saw it and tried to learn from it I got more confused. 
What do you mean by “:” where the variables are?
Is it just getting complicated or is it necessary to do all that?
Why does he do that?
My only doubt is the variables that he uses in the function, not the code.
Weird Variables Function
type FilterType = "Blacklist"|"Whitelist"
local function findPartsInGuiObject(guiObject: GuiObject, filterList: {Instance}?, filterType: FilterType?): {BasePart}
local a = guiObject.AbsolutePosition
local b = guiObject.AbsolutePosition + guiObject.AbsoluteSize
print(guiObject.AbsolutePosition, guiObject.AbsoluteSize, b)
--- make sure a is the top left and b the bottom right
a, b = Vector2.new(math.min(a.X, b.X), math.min(a.Y, b.Y)), Vector2.new(math.max(a.X, b.X), math.max(a.Y, b.Y))
filterType = filterList and filterType or "Whitelist"
filterList = filterList or (filterType == "Blacklist" and {}) or workspace:GetDescendants()
local isBlacklist = filterType == "Blacklist"
local isWhitelist = filterType == "Whitelist"
local instances: {BasePart} = {};
local rel, x, y
for _, instance: BasePart in ipairs(isWhitelist and filterList or workspace:GetDescendants()) do
local point, onScreen = camera:WorldToScreenPoint(instance.Position)
if not onScreen then
continue
end
--- if FilterType is Blacklist check if the instance is a Descendant or not and skip if so
if isBlacklist then
local skip = false
for _, blacklistedInstance: Instance in ipairs(filterList :: {Instance}) do
if instance == blacklistedInstance or instance:IsDescendantOf(blacklistedInstance) then
skip = true
break
end
end
if skip then
continue
end
end
if a.X < point.X and point.X < b.X and a.Y < point.Y and point.Y < b.Y then
table.insert(instances, instance);
end
end
return instances;
end