What does Matthew 5:18 mean?

Jesus declared in the previous verse that He had not come to abolish the Jewish law and the Prophets, but to fulfill them (Matthew 5:17). Despite claims of His critics (Matthew 12:2), Jesus' purpose was not to cast aside the words God gave to Israel. Instead, His mission was to complete the exact mission those words were meant to explain (Hebrews 8:6–8). In every way, Jesus accomplished what the law demanded and fulfilled what the prophets predicted by the power of God.

Now Jesus adds forcefully that the law will not pass away, even a little bit, until everything in it is accomplished and until heaven and earth pass away. He has come to earth (Matthew 1:18) with the mission of accomplishing everything in the law. Much of this He will do through His teaching, sacrificial death, and resurrection (Mark 8:31). The rest, He will finally and fully complete in bringing the kingdom of heaven to earth in the end times (Matthew 26:64).

Iota is the smallest letter in the Greek alphabet; modern English continues to use this as a metaphor for the tiniest measure of something. The word "dot" is translated from the Greek term keraia, referring to a tiny stroke of the pen. In Hebrew, similar tiny marks differentiated one letter from another. So, Jesus' point is that every letter of every word in the law would remain in effect until He accomplished everything in it (Hebrews 9:10–11; 10:1–4). He would do this by perfectly obeying every requirement of the law, dying as the final blood sacrifice for the sins of humanity, defeating death in the resurrection, then by fulfilling every prophecy about the promised Messiah in both His immediate time and in the future.