WhAt does the secret slab do?

Nothing really. Just a item. That is about it. Nothing really special, may be able to be sold for money, but nothing special.

Not really... It allows to meet legendary pokemon after you clear the Marine resort. You can search where to find them but here are some examples.

With secret slab in your bag:
Zapdos- 7th floor of Amp Plains
Moltres- 10th floor of Giant Volcano
Lugia- 18th floor of Surrounded Sea
Ho-oh- 19th floor of Mt. Mistral
Deoxys- 17th floor of Shimmer Hill

The Mystery Part and Secret Slab cause Pokemon to appear in dungeons they wouldn't appear in otherwise.

They give you more Legendary Pokemon to battle in Mystery Dungeons. But nothing else, really.

It is awesome I met darkai on 3rd floor of miricle sea

If you use it, it does nothing. But it will allow you to see legendaries in some dungeons. This does not, however, guarantee that they will become part of your exploration team.

I found out somthing about the secret slab when I went to the the the last florr in the dark crater I gave the secret slab to my charicter and used it some how got me back to the future!!!! If you don't beleve me try it yore self BUT IT REALY DID WORK WITH ME I'm BEIG ng.SEREOS. But theres one problem I havent found a way to get back yet also you can't get other Pokemon on yore side and yore partner dosent come with you but it firt brings you back where you start in the future when you first go there you can only go to the places you went to befor when you first go there.