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.

Here's a great post that relates to your question.

In the answers section, there are many examples provided that explain what pairs does, as well as ipairs and next.