21:36 What is Jesus alluding to here?

Jesus is alluding to the rapture here - the catching away of the church from the earth to go to Heaven with Jesus. This is the first mention of the rapture in scripture.

Being raptured is the only way anyone living at that time can escape the events of the Great Tribulation Jesus outlined in Lk 21:25-26 (cp Lk 21:25-26; Jn 14:1-3; Php 3:20-21; Col 3:1-7; 1Th 4:13-18; 2Th 2:7-12) but only those who are totally consecrated to the service of God and completely yielded to the authority of Jesus will be counted worthy to escape those things that shall come to pass, and to stand before Jesus as 2Th 2:7-12 clearly teaches (cp 1Jn 2:3-6, 15-17, 28).

See also comments on Jn 5:28-29, 14:1-3, 1Cor 15:51-58, Php 3:20-21, 1Th 4:13-18, 5:1-11, 2Th 2:6-8, 2Ti 2:18, Rev 1:19, 3:7-13, and author's study The Rapture in his book Foundational Truths of the Christian Faith.