Please explain for i, v in pairs() do?

a for i,v in pairs() do loop is a type of for loop often used for looping through tables.

An example is:

local myTable = {"my","table",5}
for i,v in pairs(myTable) do
	print(i,v)
end

The i,v is Index and Value, where the Index shows the number you're up to in the loop and the Value is the ValueType of what you're up to. Eg, if on the second go it would print out 2 table.

Another example:

local model = workspace.DoorModel
for i,v in pairs(model:GetDescendants()) do
	v.Transparency = .5 
end

This would change all the descendants of the DoorModel to have their transparency set to 0.5.