Follow

From the RuneScape Wiki, the wiki for all things RuneScape

A player may choose the "Follow" option after right-clicking another player. The "follow" option is often used when one player is using the other as an easy mode of transportation, or one is taxiing the other. The following player will proceed to follow the target player until the target player is either out of range or the user uses another option, in which case they will stop and the followed player will continue.

If two players follow each other, they would seem to circle or walk through each other. Sometimes this is purposfully started, known sometimes as a "Follow Contest", to see who breaks off first.  Also, sometimes players would form a "Follow Caravan" by following people in a row, popular amongst people waiting for Castle Wars to start.