Slp filed against an high court order

Dictionary meaning of Leave in Legal language 

Leave of court is permission from the judge to take some action in a lawsuit that requires an absence or delay. An attorney might request a leave of court in order to file an amended Pleading, a formal declaration of a claim, or a defense.

In simple english permsision granted 

Leave granted means permission granted. 

Appeal is right of the litigant but special leave petition is not. Therefore, first you request supreme court to grant leave/permission to appeal against the order and after granting leave supreme court decides your case on merits. 

Now coming to your case, Leave granted mean supreme court has allowed the petition and issued notice to the opposition. Now the opposition will file its counter affidavit and then supreme court will hear your case on merits and decide it accordingly.