Could someone explain coroutines to me, and maybe give an example?

A coroutine is essentially a new thread within a script. This allows you to have multiple loops at the same time and to continue a script while also yielding, amongst other things. For example:

local loop = coroutine.create(function()
    while true do
       wait()
       print("Hello")
    end
end)
coroutine.resume(loop)
while wait() do
    print("World!")
end
--Both loops here would run simultaneously