What is Sharah-e-Sadr or Inshirah-us-Sadr

What is Sharah-e-Sadr or Inshirah-us-Sadr

أَفَمَن شَرَحَ اللَّـهُ صَدْرَهُ لِلْإِسْلَامِ فَهُوَ عَلَىٰ نُورٍ مِّن رَّبِّهِ [Az-Zumar; 39:22] What is Sharah-e-Sadr or Inshirah-us-Sadr”,So is one whose breast Allah has expanded to [accept] Islam and he is…