What does i, v mean and work in lua in general?

Typically used for looping,

for i, v in pairs(workspace:GetChildren()) do
    print(i,v)
end

It returns each object along with a number, i is the number and v is the object.

Output from code I sent above:

Terrain Camera Baseplate

On the other hand

for i = 1,100,1 do
    print(i)
end

That would be a way to increment a value.

if you printed i, in the output it would show all the numbers up to 100, this can be useful for different effects like transparency and tweens, etc..

Note that you can use tables in for loops

local table = {"Hi", 50, "bob"}
for _, v in pairs(table) do
    print(v)
end

Hi 50 bob

Also _ is used when you don't need it, we don't need it right now because we aren't indexing a specific item in the table. If we did, the code would look like this:

local table = {"Hi", 50, "Bob"}
for i, v in pairs(table) do
    if i == 3 then
        print("Table value indexed: ",v)
    end
end

Table value indexed: Bob

This can also just be shortened to

print("Table value indexed: ",table[3])

Table value indexed: Bob