What does RenderStepped:wait() do?

RenderStepped

RenderStepped is an event of RunService. It's fired every frame render which is aproximately 1/60th of a second.

Wait

wait is a function that can wait down to aproximately 1/30th of a second.

Combining the two

When you use game:GetService('RunService').Renderstepped:wait() it makes RenderStepped redundant since the fastest the wait function can wait is apox 1/30th of a second. Just remove the wait() and have it in an event!.

Code

game:GetService('RunService').RenderStepped:connect(function()
    --code
end)

The above code would run every RenderStep, aprox 1/60th of a second.

I don't really know what you mean by combining them, but RenderStepped can only be used in local scriptsand it's basically a wait(1/60), but without RenderStepped the least you can go to is wait(1/30), you should only use RenderStepped if you want to make something run as smooth as possible. For example moving a Textlabel locally.

Note: If you only write wait() without any number in the parameters that already defaults to wait(1/30)

local rs = game:GetService("RunService").RenderStepped

for i = 1, 20 do
    rs:wait()
    local text = script.Parent 
    text.Position = text.Position + UDim2.new(0,1,0,0)
end

That would make the gui move really smoothly as nearly as if it was tweening, if not already.

Hope this helps!