Multi-leg tickets are air-tickets that fly not just from A to B but go via a number of other destinations. For example, you may want to fly to New York to stay for a few days, fly on
to Los Angeles, where you stay for three weeks before spending a week in Mexico City and flying back to London. You can book these tickets online but there are a lot of factors involved in getting the best price. The more complex the journey then the more your plans may change en-route - some agents have offices around the world making it easier to make changes to your itinerary.