"Attack and Defense" is a great theme in chess. You should know when and how you should attack, and when and how you have to defend
In the position above the attacking move 21. g5 seems to be obvious. It seems that it would open necessary files on king side. Why should White hesitate? Indeed, there is a good reason for it, because it would directly run into a decisive counter attack
So, White played here the 21. Qe4!, hindering the pawn sacrifice 21. ... e4! and keeping the black bishop out of attack.
It was obviously a necessary defending move. But not only a defending move, it was also an attacking move preparing next 22. Bd3 with a dangerous diagonal battery. And, of course, White has g5 still in reserve.
How the game went on I will show next time!