He answered a question asked in v3
The 'end' that the disciples asked about
Jesus statement is a long answer to a question asked just before.
Matthew 24:3 (NASB, emphasis added)
3 As he sat on the Mount of Olives, the disciples came to him privately, saying, “Tell us, when will these things be, and what will be the sign of your coming and of the end of the age?”
The hermeneutic here is "first mention". We could also consider it "context".
The disciples had asked him a question which, apparently, needed a small essay to answer. At the closing of Jesus's eleven-verse answer, he basically declares the close of the answer, restating part of the question by saying, "then the end will come."
Most concise answer: It is "the end" that the disciples asked him about.
The deeper question: What 'end' did the disciples refer to in asking?
That seems almost as a non-answer: "The end" is the aforementioned "end". So, two things about that:
- Good hermeneutics compel us to only extract as much meaning as a text itself has. Just because we remain curious does not mean that the text has more meaning to find.
- We can, and probably should, use simple answers like this to pursue further study.
So, the deeper question for the Bible student may be, "What 'end' were the disciples referring to when they asked Jesus."
For that, you could do research on the Eschatology of the pre-crucifixion audience. That could be either a cross-referencing study or an extrabilical-historical study.
But, the most simple answer to this "deeper" question is: The Book of Revelation. The disciples are probably asking about the same "end" John saw in his vision. In fact, John was one of the disciples originally asking. His vision at Patmos could have been a more elaborate and belated answer to his question as a young disciples, some 60 years earlier.
The Early Church including the Book of Revelation in canon seems to be the strongest evidence that John's vision was the same "end" the New Testament audience was curious about.
With the overlap of the Olivet Discourse, we could say that The Book of Revelation and the Olivet Discourse describe the same end that the disciples were curious about, which in turn Jesus describes in his answer.
From Wikipedia: Olivet Discourse
The Olivet Discourse or Olivet prophecy is a biblical passage found in the Synoptic Gospels in Matthew 24 and 25, Mark 13, and Luke 21. It is also known as the Little Apocalypse because it includes the use of apocalyptic language...
AD 70 Massacre's relevance
Some parts of the Olivet Discourse could refer to the AD 70 Massacre, otherwise known as the Siege of Jerusalem. Specifically, Jesus' reference to retrieving personal belongings in 24:17...
Matthew 24:17 (NASB)
17 Let the one who is on the housetop not go down to take what is in his house,
24:17 could refer to the AD 70 Massacre, or a future conflict in Jerusalem with the Antichrist, but 24:14 cannot.
The specific "end" Jesus refers to in 24:14 could not be the AD 70 event because he specifically says that the gospel must be preached to the nations first, then this "end" will come.
And, John's vision (the Book of Revelation) is largely agreed to have occurred around AD 90, well after. So, there certainly are references to events well beyond AD 70. That said, 24:17 could have multiple fulfillments, but 24:17 is not the full definition of this "end" the Question was about.