float object up and down

For example using the sine or the cosine function.

position = base_position + ( amplitude * sin(speed * absolute_time) )

where base_position would be the point around which it'll oscilate, amplitude is how much plus/minus around it it will oscilate (sine and cosine operate on all real numbers and return values [-1; +1]), speed scales the 'animation' speed (when 1, it'll do 1 'loop' in 2PI seconds) and absolute_time is your system time, for example.

5 hours ago, cebugdev said:

[...] bouncing up and down like say a floating/flying island.

I don't think I've ever seen a floating/flying island. Can you point us to some reference video of what you are trying to achieve?

Do you want to achieve something like a spring joint in all three axises on an object? This will couse it to bounce when interacted with a force and returning to original position when force gets balanced off.

I think OP desires something like this:

Note that you won't achieve exactly this with my solution, it'll be much more "fluid" but you can experiment with smoothstep for example.