Krishna is about to share something precious. He says: that imperishable reality which the Vedic scholars describe, which renunciants free of attachment enter, and which seekers pursue through the discipline of brahmacharya — I will explain that supreme state to you in brief.
This shloka serves as an introduction. Krishna is essentially saying: what sages gained through years of austerity, I am about to share with you right now. Like a grandfather telling his grandchild — come, let me tell you the secret that my own grandfather once told me.
"Sangrahena pravakshye" — I will tell you in brief. Krishna knows Arjuna is on a battlefield. There is no time for a long lecture. So he will give the essence, the distilled truth.