A flight with a “+” by the price indicates the new flight costs that amount more (e.g., +$29) than your original itinerary and you’ll be charged an additional $29. If there’s a “-” by the price, it means your new flight costs that amount less (e.g., -$15) and you’ll be refunded $15 to the original form of payment (if applicable) or to a flight credit.
If the flight says $0, that means the new flight costs the same as the original flight you booked on Southwest.com and you can switch without paying more or receiving a refund/flight credit.