It's obviously significant since he repeated it four times, but what actually is the significance of the phrase?
The phrase is translated from the Greek ἀναστήσω αὐτὸ (ἐν) τῇ ἐσχάτῃ ἡμέρᾳ which occurs in the following four verses:
English translation
And this is the will of the Father who sent me, that everyone who He gave me - I shall lose none of him, but rather, I shall raise him on the last day.
Textus Receptus (Estienne, 1550)
τοῦτο δέ ἐστιν τὸ θέλημα τοῦ πέμψαντός με πατρός, ἵνα πᾶν ὃ δέδωκέν μοι μὴ ἀπολέσω ἐξ αὐτοῦ ἀλλὰ ἀναστήσω αὐτὸ ἐν τῇ ἐσχάτῃ ἡμέρᾳ
English Translation
And this is the will of Him who sent me, that everyone who sees the Son and believes in him has eternal life, and I shall raise him on the last day.
Textus Receptus (Estienne, 1550)
τοῦτο δὲ ἐστιν τὸ θέλημα τοῦ πέμψαντος με, ἵνα πᾶς ὁ θεωρῶν τὸν υἱὸν καὶ πιστεύων εἰς αὐτὸν ἔχῃ ζωὴν αἰώνιον καὶ ἀναστήσω αὐτὸν ἐγὼ τῇ ἐσχάτῃ ἡμέρᾳ
English Translation
No one can come to me unless the Father who sent me draws him, and I shall raise him on the last day.
Textus Receptus (Estienne, 1550)
οὐδεὶς δύναται ἐλθεῖν πρός με ἐὰν μὴ ὁ πατὴρ ὁ πέμψας με ἑλκύσῃ αὐτόν καὶ ἐγὼ ἀναστήσω αὐτὸν τῇ ἐσχάτῃ ἡμέρᾳ
English Translation
He who eats my flesh and drinks my blood has eternal life, and I shall raise him on the last day.
Textus Receptus (Estienne, 1550)
τρώγων μου τὴν σάρκα καὶ πίνων μου τὸ αἷμα ἔχει ζωὴν αἰώνιον καὶ ἐγὼ ἀναστήσω αὐτὸν τῇ ἐσχάτῃ ἡμέρᾳ