The ringback tone is the audible indication that the call is ringing and waiting to get answered. Best practice is to answer your call immediately with a welcome message and to play some music when putting someone on hold.
Yodel also allows you to just play a ringing tone or a voice announcement after the call is answered by the bot. Please note that if you choose a standard ringback tone that is common for your region, callers may not notice that the call is already answered and active. Active calls count towards the minutes of the phone contract of the caller. Thus, unlike the ringback tone during regular calls, this one does not imply that the call has not been answered.
More information is available at https://en.wikipedia.org/wiki/Ringback_tone