ASMR is an acronym that stands for Autonomous Sensory Meridian Response. It is often described as a tingling sensation on the skin that occurs on the head, the back of the neck, or upper back. It is typically triggered by specific auditory stimuli such as whispering, crinkling of paper, turning of paper, writing or painting, fingernails tapping on a hard surface, and even the sound of someone chewing food.
ASMR is intended to invoke a very calming and relaxing feeling and listening to ASMR can be a very pleasurable experience. Many people listen to ASMR content while studying or working, and some people listen to help them fall asleep. Other people listen to simply relax or relieve stress. ASMR can be experienced in daily life or through listening to recordings of content that triggers ASMR. In daily life, an ASMR can be the sound of rain, water filling up a bath, or the rustling of leaves on a windy day.
Not everyone experiences ASMR. Early research studies have shown that nearly 75% of people surveyed experience ASMR from the sound of whispering or speaking softly, while only 50% from tapping on hard surfaces. 
For more information, check out ASMR University: https://asmruniversity.com/
 Poerio GL, Blakey E, Hostler TJ, Veltri T (2018) More than a feeling: Autonomous sensory meridian response (ASMR) is characterized by reliable changes in affect and physiology. PLoS ONE 13(6): e0196645. https://doi.org/10.1371/journal.pone.0196645