I believe the counterpoint in the next verse explains it:
Then Jesus said to them: Amen, amen I say unto you: Except you eat the flesh of the Son of man, and drink his blood, you shall not have life in you. He that eateth my flesh, and drinketh my blood, hath everlasting life: and I will raise him up in the last day. (John 6:54-55)
They lack eternal life, which John 17:3 defines:
Now this is eternal life: That they may know thee, the only true God, and Jesus Christ, whom thou hast sent.
St Augustine, a Doctor of the Church, teaches in his 26th Tractate on John
He spoke these words, not certainly to corpses, but to living men. Whereupon, lest they, understanding it to mean this life, should strive about this thing also, He going on added, Whoso eats my flesh, and drinks my blood, has eternal life. Wherefore, he that eats not this bread, nor drinks this blood, has not this life; for men can have temporal life without that, but they can noways have eternal life.