The Vietnamese word "riu riu" is often used to describe a soft, gentle sound or a low, simmering fire. It can evoke the idea of something that is quietly happening or a gentle ambiance.
While "riu riu" is a specific term, you may encounter related phrases that also describe gentle or low sounds, but "riu riu" is quite unique in its use.
In addition to describing fire and sound, "riu riu" can also be used metaphorically to indicate something that is happening slowly and steadily over time, like a process that is gradually unfolding.