Can anyone tell me what is meant by that open MPW runs are governed by a lottery.

There are now more submissions than tape-out slots (40 per run if I remember correctly), so there is a random draw out of all designs which are tape-out ready (means pass the precheck).

So , what does make my design be accepted ?

If I understood correctly, the lottery is biased so early submissions have a better chance than later ones. This means that if you don't get accepted in one run, you can just submit again for the next one as soon as the submission period starts (since your project is ready anyway) making it unlikely you miss twice in a row.