What is a foreign county warrant?

Likely, it means that he was accused of a crime outside the county jurisdiction and they will hold him if the other county wants to come get him for that charge.

It means he had an outstanding warrant from somewhere else. Another county, it would appear.

This just means that another county has a warrant for his arrest. The other county will come get him once he has either payed the bond for the recent arrest or the new charges are disposed of. These are most often warrants for violating probation. If that is the case, he needs a lawyer because the new charges will have a very significant impact on how the probation is handled. He needs a lawyer to look into all of this.

A warrant from another county. That is what it is.

It just means that he committed an offense outside the current county of detention and that a warrant was issued for his arrest. He will most likely be held for pickup.

This means he has a warrant in another county other than the one that is holding him.