What's "the rub"?

I don't want to share TMI (too much information), but...

My dad used to say a little ditty that went like this:

There was a little chigger
He wasn't any bigger
Than the point of a very fine pin
But the lump that he raises
It itches like the blazes
And that's where the rub comes in.

I need to know what "the rub" is...  😼