The Essence
Al-Qaadir is the One who possesses absolute power and capability. If He wills something to happen, nothing can prevent it. His ability is complete and completely without limit.
In the Quran
إِنَّهُ عَلَى رَجْعِهِ لَقَادِرٌ
"Indeed, He, to return him [to life], is Able (Qaadir). (Surah At-Tariq 86:8)"
In Your Duas
اللَّهُمَّ إِنِّي أَسْتَخِيرُكَ بِعِلْمِكَ وَأَسْتَقْدِرُكَ بِقُدْرَتِكَ
"O Allah, I seek Your guidance [in making a choice] by virtue of Your knowledge, and I seek ability by virtue of Your power (Qudrah)."
— Sahih al-Bukhari 1162 (Dua of Istikhara)