That's the rub phrase
That is the biggest or most difficult problem (with the situation being discussed)
A: "Why don't you buy something to eat, man?" B: "That's the rub! I don't have any money left."
A: "You really should get your life together!" B: "That's the rub, man! I don't know what to do right now."
A problem or difficult circumstance
To have to accept some troubles or worries that must be handled by oneself
Completely controlling over a scenario or situation.
This is a line from Shakespeare's Hamlet:
"To die — to sleep.
To sleep — perchance to dream: ay, there’s the rub!
For in that sleep of death what dreams may come
When we have shuffled off this mortal coil,
Must give us pause."
To be outperformed or outrun by someone, usually by a very wide margin.
You have to be quicker to finish this task or you will eat other my dust.