The idea is that the knight shoud go somewhere and 9. Nb3 is just slightly better than 9. Nf3. It allows U to finally play 10. Be3 that failed earlier cuz of Anti-English move 6...Ng4. The Boleslavsky hole on d5 is not a real weakness 
:
https://en.wikipedia.org/wiki/Isaac_Boleslavsky#Boleslavsky_hole
If the hole at d5 and the backward pawn at d6 were readily exploitable, the Najdorf would have died out about 60 years ago. Still, they are important positional features. Playing this position properly requires exquisite positional judgment, as well as familiarity with a vast body of theory.
Isaac Boleslavsky is a practically forgotten GM, but he was a very strong player and contributed a lot to chess theory. Although he did not develop the Najdorf Sicilian, he did lay the theoretical groundwork for it. He also showed the resilience of Black’s backward d pawn in the Orthodox, or Open, variations of the KID. And he very nearly became the challenger for the world championship in 1951, losing a tiebreaker to Bronstein after a drawn 12 game match and two tiebreak games.
... And he very nearly became the challenger for the world championship in 1951, losing a tiebreaker to Bronstein after a drawn 12 game match and two tiebreak games.
That match included one of my favorite opening novelties, played by Bronstein in an obscure gambit variation of the French Winawer.
It can also return to d4 at some point, if Black exchanges exf4 after White plays f2-f4.
Somebody can explain to me what are the plan with that knight on b3 in this line? Whenever I play Nb3 after e5 by black I get confused. What are the idea behind Nb3? And how u can exploit the d5 weakness. Is even a weakness for black or not? Thanks in advance.