The Essence
Al-Maani' is the Preventer and Withholder. He prevents harm from reaching His servants, and sometimes He withholds certain worldly desires from us because He knows, in His infinite wisdom, that they would harm us.
In the Quran
مَا يَفْتَحِ اللَّهُ لِلنَّاسِ مِنْ رَحْمَةٍ فَلَا مُمْسِكَ لَهَا ۖ وَمَا يُمْسِكْ فَلَا مُرْسِلَ لَهُ مِنْ بَعْدِهِ
"Whatever Allah grants to people of mercy - none can withhold it (Mumsik); and whatever He withholds - none can release it thereafter. (Surah Fatir 35:2)"
In Your Duas
اللَّهُمَّ لاَ مَانِعَ لِمَا أَعْطَيْتَ، وَلاَ مُعْطِيَ لِمَا مَنَعْتَ
"O Allah, none can prevent (Maani') what You have given, and none can give what You have prevented."
— Sahih al-Bukhari 844