A “jot” is the smallest letter in the Hebrew alphabet (yod). A “tittle” is the English word representing a small stroke or mark used to distinguish one letter from another. It could also represent a tiny decorative flourish that the calligrapher adds to a letter of a word in the Hebrew Bible as he writes it out by hand.
Jesus is referring to two of the smallest things known to ancient Israelites to show that not even the minutest detail of the old law (and the prophetic writings) is negated or voided, but all is now accomplished and fulfilled in him.
The text of Matthew suggests future fulfillment; 3 Nephi 1:25 also indicates “the law was not yet fulfilled”; but 3 Nephi 12:18 uses the past perfect tense, meaning the law had now been fulfilled.
The purpose of the old law was to lead the people of Israel to Christ, and his commandments are now before us to save us. Keeping them is the only way any of us will enter into the kingdom of heaven.