Jesus said his flesh is real food and blood real drink. What would it really mean spiritually?

Think! When a lamb is made is slaughtered, your eating of it, unknowingly signifies that it was slaughtered for your benefit.
On the contrary, if you refuse to eat of it, then that shows that you do not acknowledge its slaughter for your own benefit.

In eating Jesus' flesh and blood, it signifies that we accept Jesus' death as a benefit to us.

Yes, he died for our benefit so as to pay a debt that was to be paid by ourselves with our own lives.