Back-to-back ticketing is when a passenger books two flights to get around ticketing restrictions like Saturday night stay requirements.
Airlines don't like practices like back-to-back ticketing since passengers do it when they're trying to get around the restrictions of discounted tickets (or, more specifically the Saturday night stay requirement).
An example of back-to-back ticketing would be a business traveler trying to fly from one city to another in the middle of the week (or, at least not over the weekend, when the discount fares kick in).
Instead of purchasing a higher-priced ticket, the traveler could purchase two, separate round trip tickets but with opposite starting points.
For example, the first ticket would be a round trip booked from Boston to Houston, and the second ticket would be a round trip booked from Houston to Boston. The business traveler would then use the first leg of the first ticket (Boston to Houston) for his first flight (say, on Monday), and the first leg of the second ticket (Houston to Boston) for the return flight (say, on Wednesday). This way, the traveler's avoided paying the higher fee for mid-week travel.
If an airline discovers that a traveler has issued back-to-back tickets, it may cancel the tickets, deny boarding, issue a warning, or take some other action.