What does double travel mean?

It refers to a way of traveling by air on both the outbound and return journeys. According to the query on Bilibili, when traveling, you take a plane to your destination, and then take a plane back after traveling. Therefore, double-fly travel refers to a way of traveling by plane both ways. Double-fly travel means that the round trip is by plane instead of taking trains, long-distance buses, ships and other means of transportation.