A carry trade is the hope that currency exchange rates won’t follow interest rate parity. You have a borrowing (cheap, low interest rate) currency and an investing (high interest rate) currency. You:
Borrow the cheap currency at the (low) risk-free rate.
Convert the cheap currency to the investing currency at the spot exchange rate.
Invest the investing currency at the (high) risk-free rate.
Wait. Patiently.
Convert the investing currency plus (lots of) interest to the cheap currency at the prevailing spot rate.
Pay off the cheap currency loan.
Party with the profits.
Of course, because you haven’t locked in the future exchange rate with a forward contract, it could change against you and wipe out all of your gains. This happens from time to time.
Unfortunately, I don’t have the current Level III Schweser books, so I cannot see the table.
In general, a currency will (likely) earn a positive roll yield if the forward points are negative; i.e., the forward rate is lower than the spot rate. I say “likely” because the actual roll yield earned will depend on how the spot rate changes, which cannot be known beforehand. A positive roll yield would make you more inclined to hedge than would a negative roll yield, all else equal.